Math.random()

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函數 Math.random() 會回傳一個偽隨機小數 (pseudo-random) 介於0到1之間(包含 0,不包含1) ,大致符合數學與統計上的均勻分佈 (uniform distribution) ,您可以選定想要的數字區間,它會透過演算法被產生並且不允許使用者自行跳選或重設成特定數字。

Math.random() 所產生的偽隨機小數不符合加密學安全性要求。請勿使用於任何加密、資安相關領域。

如有加密需求建議參考Web Crypto APIwindow.crypto.getRandomValues()

語法

Math.random()

回傳值 Return value

回傳一個偽隨機小數 (pseudo-random),小數也稱浮點數; 介於0到1之間(包含 0,不包含1) 。

範例

請留意JavaScript中的數字與許多語言一樣使用 IEEE 754 floating point numbers with round-to-nearest-even behavior, the ranges claimed for the functions below (excluding the one for Math.random() itself) aren't exact. If extremely large bounds are chosen (253 or higher), it's possible in extremely rare cases to calculate the usually-excluded upper bound.

Getting a random number between 0 (inclusive) and 1 (exclusive)

function getRandom() {
  return Math.random();
}

Getting a random number between two values

This example returns a random number between the specified values. The returned value is no lower than (and may possibly equal) min, and is less than (and not equal) max.

function getRandomArbitrary(min, max) {
  return Math.random() * (max - min) + min;
}

Getting a random integer between two values

This example returns a random integer between the specified values. The value is no lower than min (or the next integer greater than min if min isn't an integer), and is less than (but not equal to) max.

function getRandomInt(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min) + min); //The maximum is exclusive and the minimum is inclusive
}

It might be tempting to use Math.round() to accomplish that, but doing so would cause your random numbers to follow a non-uniform distribution, which may not be acceptable for your needs.

Getting a random integer between two values, inclusive

While the getRandomInt() function above is inclusive at the minimum, it's exclusive at the maximum. What if you need the results to be inclusive at both the minimum and the maximum? The getRandomIntInclusive() function below accomplishes that.

function getRandomIntInclusive(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min + 1) + min); //The maximum is inclusive and the minimum is inclusive 
}

Specifications

Specification
ECMAScript (ECMA-262)
The definition of 'Math.random' in that specification.

瀏覽器相容性

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
randomChrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 3Opera Full support 3Safari Full support 1WebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 10.1Safari iOS Full support 1Samsung Internet Android Full support 1.0nodejs Full support 0.1.100

Legend

Full support  
Full support

其他參考資料