計(jì)算數(shù)組中每個(gè)元素出現(xiàn)的次數(shù)
let names = ['Alice', 'Bob', 'Tiff', 'Bruce', 'Alice'];
let nameNum = names.reduce((pre,cur)=>{
if(cur in pre){
pre[cur]++
}else{
pre[cur] = 1
}
return pre
},{})
console.log(nameNum); //{Alice: 2, Bob: 1, Tiff: 1, Bruce: 1}
判斷對(duì)象是否為數(shù)組/對(duì)象的元素/屬性：
格式：（變量 in 對(duì)象）......注意
當(dāng)“對(duì)象”為數(shù)組時(shí)闲延，“變量”指的是數(shù)組的“索引”；
當(dāng)“對(duì)象”為對(duì)象是，“變量”指的是對(duì)象的“屬性”。
數(shù)組示例：
var arr = ["a","b","2","3","str"];
var result = ("b" in arr);
var result1 = (4 in arr);
document.write(result+""); // false
document.write(result1+""); // true
對(duì)象示例：
var obj={
w:"wen",
j:"jian",
b:"bao"
}
var result=(2 in obj);
var result1=("j" in obj);
document.write(result)+""; //false
document.write(result1)+""; //true
