100个最常问的JavaScript面试问答-第3部分（共10部分）

let language = ['JavaScript', 'Dart', 'Kotlin', 'Java', 'Swift', 'Dart']
function unique_array(arr) {
   let unique_array = arr.filter(function (elem, index, self) {
       return index == self.indexOf(elem);
   });
   return unique_array
}
console.log(unique_array(language));
// Logs [ 'JavaScript', 'Dart', 'Kotlin', 'Java', 'Swift' ]

function map(arr, mapCallback) {
  // Check if the parameters passed are right.
  if (!Array.isArray(arr) || !arr.length || typeof mapCallback !== 'function') {
    return [];
    }
    else {
      let result = [];
      // Avoid mutating the original array.
      for (let i = 0, len = arr.length; i < len; i++) {
        result.push(mapCallback(arr[i], i, arr));
        // push the result of the mapCallback in the 'result' array
        }
        return result; // return the result array
    }
}

function filter(arr, filterCallback) {
  // Check if the parameters passed are right.
  if (!Array.isArray(arr) || !arr.length || typeof filterCallback !== 'function') {
    return [];
    }
    else {
      let result = [];
      // Avoid mutating the original array.
      for (let i = 0, len = arr.length; i < len; i++) {
        // check if the return value of the filterCallback is true or "truthy"
        if (filterCallback(arr[i], i, arr)) {
        // push the current item in the 'result' array if the condition is true
        result.push(arr[i]);
      }
    }
    return result; // return the result array
  }
}

function reduce(arr, reduceCallback, initialValue) {
  // Check if the parameters passed are right.
  if (!Array.isArray(arr) || !arr.length || typeof reduceCallback !== 'function'){
    return [];
  }
  else {
    // If no initialValue has been passed to the function we're gonna use the
    let hasInitialValue = initialValue !== undefined;
    let value = hasInitialValue ? initialValue : arr[0];
    // first array item as the initialValue, Start looping at index 1 if there is no
    // initialValue has been passed to the function else we start at 0 if there is an initialValue.
    for (let i = hasInitialValue ? 0 : 1, len = arr.length; i < len; i++) {
      // Then for every iteration we assign the result of the reduceCallback to the variable value.
      value = reduceCallback(value, arr[i], i, arr);
    }
    return value;
  }
}

function named() {
   // write code here
}

let show = function () {
  console.log('Anonymous function');
};
show();

function one() {
   return Array.prototype.slice.call(arguments);
}