What are the loop methods in javascript

Method: 1. for loop; 2. for in loop; 3. while loop; 4. do while loop; 5. forEach loop; 6. map(); 7. filter(); 8. some (); 9. every(); 10. reduce(); 11. for of loop, etc.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer

12 in JavaScript A loop traversal method

1. for loop

let arr = [1,2,3];
for (let i=0; i<arr.length; i++){
 console.log(i,arr[i])
}
// 0 1
// 1 2
// 2 3

for loop is the most commonly used loop tool in JS, often used for arrays Loop through.

2. for in loop

let obj = {name:'zhou',age:'**'}
for(let i in obj){
 console.log(i,obj[i])
}
// name zhou
// age **

The for in loop is mainly used to traverse ordinary objects. i represents the key value of the object, and obj[i] represents the corresponding value. When using it to iterate over an array, the same effect can be achieved in most cases, but you should not do this. This is risky, because i is output in string form, not the numeric subscript required by the array, which means In some cases, string operations will occur, resulting in data errors, such as: '52' 1 = '521' instead of the 53 we need.

In addition, when the for in loop not only traverses its own properties, it will also find the prototype, so it is best to add a judgment in the loop body, just use obj[i].hasOwnProperty(i), so as to avoid Too many unnecessary attributes are traversed.

3. while loop
Similarly traverse the cars array, first use the for loop method

let cars=["BMW","Volvo","Saab","Ford"];
let i=0;
for (;cars[i];)
{
console.log(cars[i])
i++;
};
// BMW
// Volvo
// Saab
// Ford

and then the while loop method

cars=["BMW","Volvo","Saab","Ford"];
var i=0;
while (cars[i])
{
console.log(cars[i] + "<br>")
i++;
};

us I found that they can achieve the same effect. In fact, their underlying processing is the same. However, the for loop can put the definition, conditional judgment, and increment and decrement operations into one condition for execution. The code looks more convenient, that's all. .

4. do while loop

let i = 3;
do{
 console.log(i)
 i--;
}
while(i>0)
// 3
// 2
// 1

The do while loop is a variant of the while loop. It first performs an operation and then performs a conditional judgment. If it is true Continue to perform the operation. If it is false, the loop ends.

5. Array forEach loop

let arr = [1,2,3];
arr.forEach(function(i,index){
 console.log(i,index)
})
// 1 0
// 2 1
// 3 2

forEach loop, loops through each element in the array and takes operations. There is no return value. You don’t need to know the length of the array. It has three Parameters, only the first one is required, represents the value under the current index.

Also please note that the forEach loop cannot be stopped before all elements are called. It does not have a break statement. If you must stop, you can try a try catch statement, which is to throw out when you want to force an exit. An error is caught by catch, and then returned in catch, so that the loop can be terminated. If you often use this method, it is best to customize such a forEach function in your library.

6. Array map() method

let arr = [1,2,3];
let tt = arr.map(function(i){
 console.log(i)
 return i*2;
})
// [2,4,6]

map() method returns a new array, and the elements in the array are the values ​​of the original array elements after calling the function.
Note: The map and forEach methods can only be used to traverse arrays, not ordinary objects.

7. Array filter() method

let arr = [1,2,3];
let tt = arr.filter(function(i){
 return i>1;
})
// [2,3]

The filter method is a built-in method of the Array object. It will return the filtered elements without changing the original array.

8. Array some() method

let arr = [1,2,3];
let tt = arr.some(function(i){
 return i>1;
})
// true

some() method is used to detect whether the elements in the array meet the specified conditions (provided by the function) and return a boolean value. The original array is not changed.

9. Array every() method

let arr = [1,2,3];
let tt = arr.some(function(i){
 return i>1;
})
// 检测数组中元素是否都大于1
// false

every() method is used to detect whether all elements of the array meet the specified conditions (provided through the function) and return a boolean value , does not change the original array.

10. Array reduce() method

let arr = [1,2,3];
let ad = arr.reduce(function(i,j){
 return i+j;
})
// 6

reduce() method receives a function as an accumulator, starting with each value in the array (from left to right) Reduced and finally computed to a value.

11. Array reduceRight() method

let arr = [1,2,3];
let ad = arr.reduceRight(function(i,j){
 return i+j;
})
// 6

reduceRight() method has the same function as reduce(), it starts from the end of the array forward calculate.

12. for of loop

let arr = ['name','age'];
for(let i of arr){
 console.log(i)
}
// name
// age

The for of loop is a new statement in Es6, used to replace for in and forEach. It allows you to traverse Arrays (array ), Strings (strings), Maps (mappings), Sets (sets) and other iterable (Iterable data) data structures, pay attention to its compatibility.

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