String.prototype.padStart() - JavaScript 编辑

The padStart() method pads the current string with another string (multiple times, if needed) until the resulting string reaches the given length. The padding is applied from the start of the current string.

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The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

str.padStart(targetLength [, padString])

Parameters

targetLength

The length of the resulting string once the current str has been padded. If the value is less than str.length, then str is returned as-is.

padString Optional

The string to pad the current str with. If padString is too long to stay within the targetLength, it will be truncated from the end. The default value is "" (U+0020 'SPACE').

Return value

A String of the specified targetLength with padString applied from the start.

Examples

Basic examples

'abc'.padStart(10);         // "       abc"
'abc'.padStart(10, "foo");  // "foofoofabc"
'abc'.padStart(6,"123465"); // "123abc"
'abc'.padStart(8, "0");     // "00000abc"
'abc'.padStart(1);          // "abc"

Fixed width string number conversion

// Javascript version of: (unsigned)
//  printf "%0*d" width num
function leftFillNum(num, targetLength) {
    return num.toString().padStart(targetLength, 0);
}

const num = 123;
console.log(leftFillNum(num, 5));
// expected output: "00123"

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'String.prototype.padStart' in that specification.

Browser compatibility

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See also

• String.prototype.padEnd()
• A polyfill