Implement an array in the form of JS function prototype. forEach()，. map()，. filter()

Array.prototype.forEach(),.map(),.filter() The three difference

• forEach(),map(),filter() All three implement the iteration of array elements
• forEach(),map() The difference is very small , The only difference is forEach() No return value , and map() There is a return value , Returns a new array
• filter() Returns the array filtered by the function

Array.prototype.forEach()

Use pure for Cycle writing forEach The effect of ,forEach The efficacy is shown in the figure ：

The implementation code is as follows ：

Array.prototype.forEach = function(callback, thisArg) {
    
  // Determine whether the first parameter is a function 
  if(typeof callback != 'function') throw new TypeError(callback + 'must be a function')

  var arr = this
  // Whether the second parameter is received 
  var thisValue = arguments[1] || this

  for(let i = 0; i < arr.length; ++i) {
    
    // change this The direction of 
    callback.call(thisValue, arr[i], i, arr)
  }
}

Array.prototype.map()

map And forEach The effect is similar ,map The efficacy is as follows ：

The code is as follows ：

Array.prototype.map = function(callback, thisArg) {
    
  // Determine whether the first parameter is a function 
  if(typeof callback != 'function') throw new TypeError(callback + 'must be a function')

  var arr = this
  // Whether the second parameter is received 
  var thisValue = arguments[1] || this
  
  // Save return value 
  let returnValue = []
  for(let i = 0; i < arr.length; ++i) {
    
    // change this The direction of + Push the return value 
    if(callback.call(thisValue, arr[i], i, arr))
    returnValue.push(arr[i])
  }
  return returnValue
}

Array.prototype.filter()

filter() The efficacy is as follows ：

The code is as follows ：

Array.prototype.filter = function(callback, thisArg) {
    
  // Determine whether the first parameter is a function 
  if(typeof callback != 'function') throw new TypeError(callback + 'must be a function')

  var arr = this
  // Whether the second parameter is received 
  var thisValue = arguments[1] || this
  
  // Save return value 
  let returnValue = []
  for(let i = 0; i < arr.length; ++i) {
    
    // The value matches the original array 
    if(callback.call(thisValue, arr[i], i, arr))
      returnValue.push(arr[i])
  }
  return returnValue
}