How are Leading Zeros in JavaScript Numbers Interpreted?

Numbers with leading zeros in JavaScript

Numbers with leading zeros in JavaScript are interpreted differently depending on their format.

Historical Evolution

Early on, leading zeros represented octal (base 8) in JavaScript, similar to the leading 0x for hexadecimal (base 16).

10, 012, 0xA // 均表示十进制数 10

In ECMAScript 3, however, this octal representation was demoted to an optional extension, and decimal integer literals can no longer have leading zeros (unless the implementation supports the extension).

Strict mode in ES5 and later

In strict mode in ES5, decimal literals with leading zeros are no longer allowed:

0 // 无效
NonZeroDigit DecimalDigits(opt) // 有效

ES6 New representations introduced

ES6 introduces binary and octal literals, with leading symbols 0b and 0o:

• Binary literal: 0b11110010
• Octal literal: 0o764

Traditional octal literal

Although no longer recommended, with Octal representations with leading zeros (i.e. traditional octal literals) can still be used in non-strict mode:

010 // 可能会被解释为 8 或 10，具体取决于实现

Best Practices

To ensure that numbers are always Interpreted as decimal, it is recommended to remove leading zeros or use parseInt parses numbers:

parseInt('010', 8) // 返回 8
parseInt('010', 10) // 返回 10

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