js array method
filter array and return one field
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| /**
* filter array and return one field
*/
let arr = [
{
name: "jay",
age: 18
},
{
name: "mike",
age: 20
},
{
name: "jack",
age: 21
}
];
// use filter and map
// two loop
let names1 = arr.filter(item => item.age >= 20).map(item => item.name);
// use reduce
// one loop
let names2 = arr.reduce(
(acc, current) => (current.age >= 20 ? [...acc, current.name] : acc),
[]
);
console.log(names1);
console.log(names2);
|
Array.from Array.of
arrayLike 对象的 key 为正整数或者０，且有 lenght 属性。函数的 arguments 和 dom 元素集。使用数组的 slice 方法也可以转成数组。
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| Array.from({length:3},(item,index)=>index);//[0,1,2]
|
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| var arr = new Array(2);
console.log(arr.length);//2
arr[0];//undefined
arr[1];//undefined
var arr = new Array(1,2);
console.log(arr.length);//2
arr[0];//undefined
arr[1];//undefined
|
Array 构造函数的传参不同，结果也不同。
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| var arr = Array.of(2);
console.log(arr.length);//1
arr[0];//2
arr[1];//undefined
var arr = Array.of(1,2);
console.log(arr.length);//2
arr[0];//1
arr[1];//2
|
