Advanced for loop writing method

1. Commonly used writing methods:

for(var i=0;i<arr.length;i++){
console.log(i);
console.log(arr[i]);
}

You need to read arr.length once every time to make a judgment

2,

for(var i=0,len=arr.length;i<len;i++){
console.log(i);
console.log(arr[i]);
}

The variables are completely defined, there is no need to retrieve the length from the array, and the computer can directly determine i and length

3,

for(var i=0,val;val=arr[i++];){
console.log(i);
console.log(val);
}

Put judgment and assignment together, and assign values ​​while looping;

Look at another for(var i=10;i--;){console.log(i);}

At first glance, you may think it is wrong because the statement is not complete, but the loop will be 10 times. Why?

Because the second sentence of the conditional sentence i<10; returns true; and in js, 0, null, undefined, false and empty string will all be false values

So, in i When it becomes 0, it is automatically converted to a Boolean value false, thereby ending the loop;

If you understand this, then look back at the third way of writing above,

for(var i = 0;i<arr.length;i++){
var val = arr[i];
}

i = 0 and var at the same time Put in the initial condition, val = arr[i++] is judged. If arr[i++]>arr.length, then val is undefined, the judgment is undefined, the judgment is terminated, and the loop ends;

Therefore, the loop You can use abbreviations such as 0, null, undefined, false and empty strings to make judgments and learn programming thinking.

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