TypedArray.from()

TypedArray.from() 方法 从一个类数组或者可迭代对象中创建一个新类型数组。 这个方法和 Array.from()类似。

语法

TypedArray.from(source[, mapFn[, thisArg]])

where TypedArray is one of:

Int8Array
Uint8Array
Uint8ClampedArray
Int16Array
Uint16Array
Int32Array
Uint32Array
Float32Array
Float64Array

参数

source
想要转换为类型数组的类数组或者可迭代对象。
mapFn
可选参数。如果指定了该参数,则最后生成的类型数组会经过该函数的加工处理后再返回。
thisArg
可选参数。执行  mapFn 函数时  this 的值。

返回值

一个新的 TypedArray 实例。

 描述

TypedArray.from() 允许你从下面两者来创建数组:

  • 类数组对象(拥有一个 length 属性和若干索引属性的任意对象)
  • Set 等)。

TypedArray.from() 方法有一个可选参数 mapFn, 让你可以在最后生成的类型数组上再执行一次 map 方法后再返回。也就是说 TypedArray.from(obj, mapFn, thisArg) 和TypedArray.from(Array.prototype.map.call(obj, mapFn, thisArg)) 是等价的。

 from()length 属性为1.

Array.from() 和 TypedArray.from()之间有一些微妙的区别:

  • 如果 |this| 的值传递给 TypedArray.from 不是一个构造器, TypedArray.from 将抛出{jsxref("TypeError")}}, 而 Array.from 默认将创建一个 Array.
  • TypedArray.from 使用[Put] 而 rray.from 使用[DefineProperty]]. 因此, 当和 handler.defineProperty.
  • 当 from 获得一个迭代器时, TypedArray 一开始收集迭代器中的所有值, 此时创建一个 |this| 的实例用于计数, 然后在实例中设置值。 Array.from 设置每个从迭代器其中获取的值,最后设置它的长度。
  • 当 Array.from 获得一个不可迭代的类数组时, it respects holes, 而 TypedArray.from 将确保结果是 dense.

示例

// Set (iterable object)
var s = new Set([1, 2, 3]);
Uint8Array.from(s);
// Uint8Array [ 1, 2, 3 ]


// String
Int16Array.from('123');                      
// Int16Array [ 1, 2, 3 ]


// Using an arrow function as the map function to
// manipulate the elements
Float32Array.from([1, 2, 3], x => x + x);      
// Float32Array [ 2, 4, 6 ]


// Generate a sequence of numbers
Uint8Array.from({length: 5}, (v, k) => k);    
// Uint8Array [ 0, 1, 2, 3, 4 ]

Polyfill 

You can partially work around this by inserting the following code at the beginning of your scripts, allowing use of much of the functionality of from() in implementations that do not natively support it.

if (!Int8Array.__proto__.from) {
    (function () {
        Int8Array.__proto__.from = function (obj, func, thisObj) {

            var typedArrayClass = Int8Array.__proto__;
            if(typeof this !== 'function') {
                throw new TypeError('# is not a constructor');
            }
            if (this.__proto__ !== typedArrayClass) {
                throw new TypeError('this is not a typed array.');
            }
 
            func = func || function (elem) {
                    return elem;
                };

            if (typeof func !== 'function') {
                throw new TypeError('specified argument is not a function');
            }

            obj = Object(obj);
            if (!obj['length']) {
                return new this(0);
            }
            var copy_data = [];
            for(var i = 0; i < obj.length; i++) {
                copy_data.push(obj[i]);
            }

            copy_data = copy_data.map(func, thisObj);

            var typed_array = new this(copy_data.length);
            for(var i = 0; i < typed_array.length; i++) {
                typed_array[i] = copy_data[i];
            }
            return typed_array;
        }
    })();
}

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
%TypedArray%.from
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
%TypedArray%.from
Draft  

Browser compatibility

Update compatibility data on GitHub
Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js
from Chrome Full support 45 Edge Full support 14 Firefox Full support 38 IE No support No Opera No support No Safari Full support 10 WebView Android No support No Chrome Android No support No Firefox Android Full support 38 Opera Android No support No Safari iOS Full support 10 Samsung Internet Android No support No nodejs Full support 4.0.0

Legend

Full support  
Full support
No support  
No support

See also