Detailed usage of js array forEach instance

1. forEach() is similar to map(). It also applies each element to the passed function in turn, but does not return a new array.

2. forEach() is often used to traverse an array, calling each element of the array and passing it to the callback function. Transfer functions do not need to return a value.

Examples

var arr = [7,4,6,51,1];
try{arr.forEach((item,index)=>{
      if (item<5) {
       throw new Error("myerr") //Create a new error message for myerr
      }
      console.log(item)//Only print 7 to indicate that the loop has been exited})}catch(e){
            console.log(e.message);
      if (e.message!=="myerr") { //If it is not the error we defined, just throw it away. throw e
      }
     }

Knowledge point expansion:

Handwritten forEach

forEach() method executes a provided function once for each element of an array

arr.forEach(callback(currentValue [, index [, array]])[, thisArg]);

• callback

  • currentValue
    The current element being processed in the array.
  • index Optional The index within the array of the current element being processed.
  • array optional
    The array that the forEach() method is operating on.
  • thisArg OptionalOptional parameter. Used as the value of this when the callback function is executed.
• No return value

If a thisArg parameter is provided to the forEach function, the parameter will be used as this value in the callback function. Otherwise the value of this is undefined . The binding of this in the callback function is determined according to the general this binding rules when the function is called.

let arr = [1, 2, 3, 4];

arr.forEach((...item) => console.log(item));

// [1, 0, Array(4)] Current value

function Counter() {
 this.sum = 0;
 this.count = 0;
}

// Because the thisArg parameter (this) is passed to forEach(), each time it is called, it is passed to the callback function as its this value.
Counter.prototype.add = function(array) {
 array.forEach(function(entry) {
  this.sum += entry;
  ++this.count;
 }, this);
 // ^---- Note
};

const obj = new Counter();
obj.add([2, 5, 9]);
obj.count;
// 3 === (1 + 1 + 1)
obj.sum;
// 16 === (2 + 5 + 9)

• Every array has this method
• The callback parameters are: each item, index, original array

Array.prototype.forEach = function(fn, thisArg) {
 var _this;
 if (typeof fn !== "function") {
  throw "the parameter must be a function";
 }
 if (arguments.length > 1) {
  _this = thisArg;
 }
 if (!Array.isArray(arr)) {
  throw "forEach method can only be used on arrays";
 }

 for (let index = 0; index < arr.length; index++) {
  fn.call(_this, arr[index], index, arr);
 }
};

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