Expressions and operators - JavaScript 编辑

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This chapter describes JavaScript's expressions and operators, including assignment, comparison, arithmetic, bitwise, logical, string, ternary and more.

A complete and detailed list of operators and expressions is also available in the reference.

Operators

JavaScript has the following types of operators. This section describes the operators and contains information about operator precedence.

• Assignment operators
• Comparison operators
• Arithmetic operators
• Bitwise operators
• Logical operators
• String operators
• Conditional (ternary) operator
• Comma operator
• Unary operators
• Relational operators

JavaScript has both binary and unary operators, and one special ternary operator, the conditional operator. A binary operator requires two operands, one before the operator and one after the operator:

operand1 operator operand2

For example, 3+4 or x*y.

A unary operator requires a single operand, either before or after the operator:

operator operand

or

operand operator

For example, x++ or ++x.

Assignment operators

An assignment operator assigns a value to its left operand based on the value of its right operand. The simple assignment operator is equal (=), which assigns the value of its right operand to its left operand. That is, x = y assigns the value of y to x.

There are also compound assignment operators that are shorthand for the operations listed in the following table:

Return value and chaining

Like most expressions, assignments like x = y have a return value. It can be retrieved by e.g. assigning the expression or logging it:

const z = (x = y); // Or equivalently: const z = x = y;

console.log(z); // Log the return value of the assignment x = y.
console.log(x = y); // Or log the return value directly.

The return value matches the expression to the right of the = sign in the “Meaning” column of the table above. That means that (x = y) returns y, (x += y) returns the resulting sum x + y, (x **= y) returns the resulting power x ** y, and so on.

In the case of logical assignments, (x &&= y), (x ||= y), and (x ??= y), the return value is that of the logical operation without the assignment, so x && y, x || y, and x ?? y, respectively.

Note that the return values are always based on the operands’ values before the operation.

When chaining these expressions, each assignment is evaluated right-to-left. Consider these examples:

• w = z = x = y is equivalent to w = (z = (x = y)) or x = y; z = y; w = y
• z += x *= y is equivalent to z += (x *= y) or tmp = x * y; x *= y; z += tmp (except without the tmp).

Destructuring

For more complex assignments, the destructuring assignment syntax is a JavaScript expression that makes it possible to extract data from arrays or objects using a syntax that mirrors the construction of array and object literals.

var foo = ['one', 'two', 'three'];

// without destructuring
var one   = foo[0];
var two   = foo[1];
var three = foo[2];

// with destructuring
var [one, two, three] = foo;

Comparison operators

A comparison operator compares its operands and returns a logical value based on whether the comparison is true. The operands can be numerical, string, logical, or object values. Strings are compared based on standard lexicographical ordering, using Unicode values. In most cases, if the two operands are not of the same type, JavaScript attempts to convert them to an appropriate type for the comparison. This behavior generally results in comparing the operands numerically. The sole exceptions to type conversion within comparisons involve the === and !== operators, which perform strict equality and inequality comparisons. These operators do not attempt to convert the operands to compatible types before checking equality. The following table describes the comparison operators in terms of this sample code:

var var1 = 3;
var var2 = 4;

Note: (=>) is not an operator, but the notation for Arrow functions.

Arithmetic operators

An arithmetic operator takes numerical values (either literals or variables) as their operands and returns a single numerical value. The standard arithmetic operators are addition (+), subtraction (-), multiplication (*), and division (/). These operators work as they do in most other programming languages when used with floating point numbers (in particular, note that division by zero produces Infinity). For example:

1 / 2; // 0.5
1 / 2 == 1.0 / 2.0; // this is true

In addition to the standard arithmetic operations (+, -, *, /), JavaScript provides the arithmetic operators listed in the following table:

Bitwise operators

A bitwise operator treats their operands as a set of 32 bits (zeros and ones), rather than as decimal, hexadecimal, or octal numbers. For example, the decimal number nine has a binary representation of 1001. Bitwise operators perform their operations on such binary representations, but they return standard JavaScript numerical values.

The following table summarizes JavaScript's bitwise operators.

Bitwise logical operators

Conceptually, the bitwise logical operators work as follows:

• The operands are converted to thirty-two-bit integers and expressed by a series of bits (zeros and ones). Numbers with more than 32 bits get their most significant bits discarded. For example, the following integer with more than 32 bits will be converted to a 32 bit integer:

  Before: 1110 0110 1111 1010 0000 0000 0000 0110 0000 0000 0001
  After:               1010 0000 0000 0000 0110 0000 0000 0001

• Each bit in the first operand is paired with the corresponding bit in the second operand: first bit to first bit, second bit to second bit, and so on.
• The operator is applied to each pair of bits, and the result is constructed bitwise.

For example, the binary representation of nine is 1001, and the binary representation of fifteen is 1111. So, when the bitwise operators are applied to these values, the results are as follows:

Note that all 32 bits are inverted using the Bitwise NOT operator, and that values with the most significant (left-most) bit set to 1 represent negative numbers (two's-complement representation). ~x evaluates to the same value that -x - 1 evaluates to.

Bitwise shift operators

The bitwise shift operators take two operands: the first is a quantity to be shifted, and the second specifies the number of bit positions by which the first operand is to be shifted. The direction of the shift operation is controlled by the operator used.

Shift operators convert their operands to thirty-two-bit integers and return a result of the same type as the left operand.

The shift operators are listed in the following table.

Logical operators

Logical operators are typically used with Boolean (logical) values; when they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they may return a non-Boolean value. The logical operators are described in the following table.

Examples of expressions that can be converted to false are those that evaluate to null, 0, NaN, the empty string (""), or undefined.

The following code shows examples of the && (logical AND) operator.

var a1 =  true && true;     // t && t returns true
var a2 =  true && false;    // t && f returns false
var a3 = false && true;     // f && t returns false
var a4 = false && (3 == 4); // f && f returns false
var a5 = 'Cat' && 'Dog';    // t && t returns Dog
var a6 = false && 'Cat';    // f && t returns false
var a7 = 'Cat' && false;    // t && f returns false

The following code shows examples of the || (logical OR) operator.

var o1 =  true || true;     // t || t returns true
var o2 = false || true;     // f || t returns true
var o3 =  true || false;    // t || f returns true
var o4 = false || (3 == 4); // f || f returns false
var o5 = 'Cat' || 'Dog';    // t || t returns Cat
var o6 = false || 'Cat';    // f || t returns Cat
var o7 = 'Cat' || false;    // t || f returns Cat

The following code shows examples of the ! (logical NOT) operator.

var n1 = !true;  // !t returns false
var n2 = !false; // !f returns true
var n3 = !'Cat'; // !t returns false

Short-circuit evaluation

As logical expressions are evaluated left to right, they are tested for possible "short-circuit" evaluation using the following rules:

• false && anything is short-circuit evaluated to false.
• true || anything is short-circuit evaluated to true.

The rules of logic guarantee that these evaluations are always correct. Note that the anything part of the above expressions is not evaluated, so any side effects of doing so do not take effect.

Note that for the second case, in modern code you can use the new Nullish coalescing operator (??) that works like ||, but it only returns the second expression, when the first one is "nullish", i.e. null or undefined. It is thus the better alternative to provide defaults, when values like '' or 0 are valid values for the first expression, too.

String operators

In addition to the comparison operators, which can be used on string values, the concatenation operator (+) concatenates two string values together, returning another string that is the union of the two operand strings.

For example,

console.log('my ' + 'string'); // console logs the string "my string".

The shorthand assignment operator += can also be used to concatenate strings.

For example,

var mystring = 'alpha';
mystring += 'bet'; // evaluates to "alphabet" and assigns this value to mystring.

Conditional (ternary) operator

The conditional operator is the only JavaScript operator that takes three operands. The operator can have one of two values based on a condition. The syntax is:

condition ? val1 : val2

If condition is true, the operator has the value of val1. Otherwise it has the value of val2. You can use the conditional operator anywhere you would use a standard operator.

For example,

var status = (age >= 18) ? 'adult' : 'minor';

This statement assigns the value "adult" to the variable status if age is eighteen or more. Otherwise, it assigns the value "minor" to status.

Comma operator

The comma operator (,) evaluates both of its operands and returns the value of the last operand. This operator is primarily used inside a for loop, to allow multiple variables to be updated each time through the loop. It is regarded bad style to use it elsewhere, when it is not necessary. Often two separate statements can and should be used instead.

For example, if a is a 2-dimensional array with 10 elements on a side, the following code uses the comma operator to update two variables at once. The code prints the values of the diagonal elements in the array:

var x = [0,1,2,3,4,5,6,7,8,9]
var a = [x, x, x, x, x];

for (var i = 0, j = 9; i <= j; i++, j--)
//                                ^
  console.log('a[' + i + '][' + j + ']= ' + a[i][j]);

Unary operators

A unary operation is an operation with only one operand.

delete

The delete operator deletes an object's property. The syntax is:

delete object.property;
delete object[propertyKey];
delete objectName[index];
delete property; // legal only within a with statement

where object is the name of an object, property is an existing property, and propertyKey is a string or symbol referring to an existing property.

The fourth form is legal only within a with statement, to delete a property from an object, and also for properties of the global object.

If the delete operator succeeds, it removes the property from the object. Trying to access it afterwards will yield undefined. The delete operator returns true if the operation is possible; it returns false if the operation is not possible.

x = 42; // implicitly creates window.x
var y = 43;
var myobj = {h: 4}; // create object with property h

delete x;       // returns false (cannot delete if created implicitly)
delete y;       // returns false (cannot delete if declared with var)
delete Math.PI; // returns false (cannot delete non-configurable properties)
delete myobj.h; // returns true (can delete user-defined properties)

Deleting array elements

Since arrays are just objects, it's technically possible to delete elements from them. This is however regarded as a bad practice, try to avoid it. When you delete an array property, the array length is not affected and other elements are not re-indexed. To achieve that behavior, it is much better to just overwrite the element with the value undefined. To actually manipulate the array, use the various array methods such as splice.

typeof

The typeof operator is used in either of the following ways:

typeof operand
typeof (operand)

The typeof operator returns a string indicating the type of the unevaluated operand. operand is the string, variable, keyword, or object for which the type is to be returned. The parentheses are optional.

Suppose you define the following variables:

var myFun = new Function('5 + 2');
var shape = 'round';
var size = 1;
var foo = ['Apple', 'Mango', 'Orange'];
var today = new Date();

The typeof operator returns the following results for these variables:

typeof myFun;       // returns "function"
typeof shape;       // returns "string"
typeof size;        // returns "number"
typeof foo;         // returns "object"
typeof today;       // returns "object"
typeof doesntExist; // returns "undefined"

For the keywords true and null, the typeof operator returns the following results:

typeof true; // returns "boolean"
typeof null; // returns "object"

For a number or string, the typeof operator returns the following results:

typeof 62;            // returns "number"
typeof 'Hello world'; // returns "string"

For property values, the typeof operator returns the type of value the property contains:

typeof document.lastModified; // returns "string"
typeof window.length;         // returns "number"
typeof Math.LN2;              // returns "number"

For methods and functions, the typeof operator returns results as follows:

typeof blur;        // returns "function"
typeof eval;        // returns "function"
typeof parseInt;    // returns "function"
typeof shape.split; // returns "function"

For predefined objects, the typeof operator returns results as follows:

typeof Date;     // returns "function"
typeof Function; // returns "function"
typeof Math;     // returns "object"
typeof Option;   // returns "function"
typeof String;   // returns "function"

void

The void operator is used in either of the following ways:

void (expression)
void expression

The void operator specifies an expression to be evaluated without returning a value. expression is a JavaScript expression to evaluate. The parentheses surrounding the expression are optional, but it is good style to use them.

Relational operators

A relational operator compares its operands and returns a Boolean value based on whether the comparison is true.

in

The in operator returns true if the specified property is in the specified object. The syntax is:

propNameOrNumber in objectName

where propNameOrNumber is a string, numeric, or symbol expression representing a property name or array index, and objectName is the name of an object.

The following examples show some uses of the in operator.

// Arrays
var trees = ['redwood', 'bay', 'cedar', 'oak', 'maple'];
0 in trees;        // returns true
3 in trees;        // returns true
6 in trees;        // returns false
'bay' in trees;    // returns false (you must specify the index number,
                   // not the value at that index)
'length' in trees; // returns true (length is an Array property)

// built-in objects
'PI' in Math;          // returns true
var myString = new String('coral');
'length' in myString;  // returns true

// Custom objects
var mycar = { make: 'Honda', model: 'Accord', year: 1998 };
'make' in mycar;  // returns true
'model' in mycar; // returns true

instanceof

The instanceof operator returns true if the specified object is of the specified object type. The syntax is:

objectName instanceof objectType

where objectName is the name of the object to compare to objectType, and objectType is an object type, such as Date or Array.

Use instanceof when you need to confirm the type of an object at runtime. For example, when catching exceptions, you can branch to different exception-handling code depending on the type of exception thrown.

For example, the following code uses instanceof to determine whether theDay is a Date object. Because theDay is a Date object, the statements in the if statement execute.

var theDay = new Date(1995, 12, 17);
if (theDay instanceof Date) {
  // statements to execute
}

Operator precedence

The precedence of operators determines the order they are applied when evaluating an expression. You can override operator precedence by using parentheses.

The following table describes the precedence of operators, from highest to lowest.

A more detailed version of this table, complete with links to additional details about each operator, may be found in JavaScript Reference.

Expressions

An expression is any valid unit of code that resolves to a value.

Every syntactically valid expression resolves to some value but conceptually, there are two types of expressions: with side effects (for example: those that assign value to a variable) and those that in some sense evaluate and therefore resolve to a value.

The expression x = 7 is an example of the first type. This expression uses the = operator to assign the value seven to the variable x. The expression itself evaluates to seven.

The code 3 + 4 is an example of the second expression type. This expression uses the + operator to add three and four together without assigning the result, seven, to a variable.

JavaScript has the following expression categories:

• Arithmetic: evaluates to a number, for example 3.14159. (Generally uses arithmetic operators.)
• String: evaluates to a character string, for example, "Fred" or "234". (Generally uses string operators.)
• Logical: evaluates to true or false. (Often involves logical operators.)
• Primary expressions: Basic keywords and general expressions in JavaScript.
• Left-hand-side expressions: Left values are the destination of an assignment.

Primary expressions

Basic keywords and general expressions in JavaScript.

this

Use the this keyword to refer to the current object. In general, this refers to the calling object in a method. Use this either with the dot or the bracket notation:

this['propertyName']
this.propertyName

Suppose a function called validate validates an object's value property, given the object and the high and low values:

function validate(obj, lowval, hival) {
  if ((obj.value < lowval) || (obj.value > hival))
    console.log('Invalid Value!');
}

You could call validate in each form element's onChange event handler, using this to pass it to the form element, as in the following example:

<p>Enter a number between 18 and 99:</p>
<input type="text" name="age" size=3 onChange="validate(this, 18, 99);">

Grouping operator

The grouping operator ( ) controls the precedence of evaluation in expressions. For example, you can override multiplication and division first, then addition and subtraction to evaluate addition first.

var a = 1;
var b = 2;
var c = 3;

// default precedence
a + b * c     // 7
// evaluated by default like this
a + (b * c)   // 7

// now overriding precedence
// addition before multiplication
(a + b) * c   // 9

// which is equivalent to
a * c + b * c // 9

Left-hand-side expressions

Left values are the destination of an assignment.

new

You can use the new operator to create an instance of a user-defined object type or of one of the built-in object types. Use new as follows:

var objectName = new objectType([param1, param2, ..., paramN]);

super

The super keyword is used to call functions on an object's parent. It is useful with classes to call the parent constructor, for example.

super([arguments]); // calls the parent constructor.
super.functionOnParent([arguments]);

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