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前端面试题2021及答案 js反转数组的方法

备忘单是我们开发人员始终需要的参考。因此,这里我编译了许多JavaScript参考代码。查看分类并找到它。这篇文章对学习者和开发人员都有帮助。

终极JavaScript秘籍,建议收藏食用


JavaScript Number方法备忘单

  • toExponential():以字符串形式返回表示Number对象的字符串
  function expo(x, f) {
      return 
      Number.parseFloat(x).toExponential(f);
  }

  console.log(expo(123456, 2)); 
  // -> 1.23e+5
  • toFixed():使用定点表示法格式化数字
  function financial(x) {
      return Number.parseFloat(x).toFixed(2); 
  }

  console.log(financial(123.456)); 
  // -> 123.46
  • toPrecision():以指定的精度返回表示Number对象的字符串
  function precise(x) {
      return
      Number.parseFloat(x).toPrecision(4); 
  }

  console.log(precise(123.456)); 
  // -> 123.5
  • toString():返回表示指定Number对象的字符串
  function hexColour(c) {
      if (c < 256) {
          return Math.abs(c).toString(16); 
      }
      return 0; 
  }

  console.log(hexColour(233)); 
  // -> e9
  • valueOf():返回数字对象的包装原始值
  const numObj = new Number(42); 
  console.log(typeof numObj); 
  // -> object

  const num = numObj.valueOf(); 
  console.log(num); 
  // -> 42

  console.log(typeof num); 
  // -> number


JavaScript循环备忘单

  • 对于循环
  for (var i = 0; < 10; i++) {
      console.log(i + ": " + i * 3 + "<br />"); 
  }
  // -> 0: 0<br />
  // -> 1: 3<br />
  // -> ...

  let a = [1, 2, 3]; 
  var sum = 0; 
  for (var i - 0; i <a.length; i++) {
      sum += a[i]; 
  } // pasing an array
  console.log(sum); 
  // -> 6
  • While循环
  var i = 1;                  // initialize
  while (i < 100) {          // enters the cycle if statement is true
      i *= 2;                 // increment to avoid infinte loop 
      console.log(i + ", "); // output
  } 
  // 2, 
  // 4, 
  // ...
  // 128, 
  • 循环执行
  var i = 1;                  // initialize
  while (i < 100) {          // enters the cycle asleast once
      i *= 2;                 // increment to avoid infinte loop 
      console.log(i + ", "); // output
  } while (1 < 100); // repeats cycle if statement is true at the end
  // 2, 
  // 4, 
  // ...
  // 128,
  • 打破
  for (var i = 0; i < 10; i++) {
      if (i == 5 ) { break; } // stops and exits the cycle
      console.log(i + ", ");  // Lat output number is 4
  }
  // -> 0, 
  // -> 1, 
  // ...
  // -> 4, 
  • 继续
  for (var i = 0; i < 10; i++) {
      if (i == 5 ) { continue; } // skips the rest of the cycle
      console.log(i + ", ");  // skips 5
  }
  // -> 0, 
  // -> 1, 
  // ...
  // -> 9,

JavaScript字符串方法备忘单

  • charAt():返回指定索引处的字符
  const sentence = "Jeff bezos is now the second richest."; 

  const index = 4; 

  console.log(`The character at index ${index} is ${sentence.charAt(index)}`); 
  // The character at index 4 is f
  • concat():连接两个或多个字符串,并返回所连接字符串的副本
  const str1 = "Hello"; 
  cosnt str2 = "World"; 

  console.log(str1.concat(" ", str2)); 
  // -> Hello World

  console.log(str2.concat(", ", str1)); 
  // -> World, Hello
  • replace():搜索子字符串(或正则表达式)和字符串之间的匹配项,并将匹配的子字符串替换为新的子字符串
  const p = "Talk is cheap. Show me the work. - Someone"; 

  console.log(p.replace("work", "code")); 
  // -> Talk is cheap. Show me the code. - Someone
  • search():搜索正则表达式和字符串之间的匹配项,并返回匹配项的位置
  const paragraph = "The quick brown fox jumps over the lazy dog."; 

  // any character that is not a word character or whitespace
  const regex = /[^ws]/g;

  console.log(paragraph.search(regex)); 
  // -> 43
  • slice():提取字符串的一部分并返回新的字符串
  const str = "The quick brown fox jumps over the lazy dog."; 

  consolelog(str.slice(31)); 
  // -> the lazy dog

  console.log(str.slice(4, 19)); 
  // -> quick brown fox
  • trim():删除字符串两端的空格
  const greeting = "  Hello world!   "; 

  console.log(greeting); 
  // -> Hello world!

  console.log(greeting.trim()); 
  // -> Hello world!
  • substr():从字符串中提取字符,从指定的起始位置开始,直到指定的字符数
  const str = "Mozilla"; 

  console.log(str.substr(1, 2)); 
  // -> oz

  console.log(stre.substr(2)); 
  // -> zilla
  • toLowerCase():将字符串转换为小写字母
  const sentence = "Elon became the richest last night."; 

  console.log(sentence.toLowerCase()); 
  // -> elon became the richest last night.

JavaScript数组方法指导表

  • concat():连接两个或多个数组,并返回联接数组的副本
  let array1 = ["a", "b", "c"]; 
  let array2 = ["d", "e", "f"]; 
  let array3 = array1.concat(array2); 

  console.log(array3); 
  // -> Array(6) ["a", "b", "c", "d", "e", "f" ]
  • indexOf():在数组中搜索元素并返回其位置
  let beasts = ["ant", "bison", "camel", "duck", "bison"]; 

  console.log(beasts.indexOf("bison")); 
  // -> 1

  // start from index 2
  console.log(beasts.indexOf("bison", 2)); 
  // -> 4
  • join():将数组的所有元素连接到一个字符串中
  let elements = ["Fire", "Air", "Water"]; 

  console.log(elements.join()); 
  // -> Fire,Air,Water

  console.log(elements.join(" ")); 
  // -> Fire Air Water
  • pop():删除数组的最后一个元素,并返回该元素
  let plants = ["broccoli", "cauliflower", "cabbage", "kale", "tomato"]; 

  console.log(plants.pop()); 
  // -> tomato

  console.log(plants); 
  // -> Array(4) ["brocxoli", "cauliflower", "cabbage", "kale"]
  • reverse():反转数组中元素的顺序
  let array1 = ["one", "two", "three"]; 
  console.log("array1:", array1); 
  // -> array1: Array(3) [ "one", "two", "three" ]

  let reversed = array1.reverse(); 
  console.log("reversed", reversed); 
  // -> reversed: Array(3) [ "three", "two", "one" ]
  • shift():删除数组的第一个元素,并返回该元素
  let array1 = [1, 2, 3]; 

  let firstElement = array1.shift(); 

  console.log(array1); 
  // -> Array [ 2, 3 ]
  • sort():对数组的元素进行排序
  let months = ["March", "Jan", "Feb", "Dec"]; 
  months.sort(); 

  console.log(months); 
  // -> Array(4) [ "Dec", "Feb", "Jan", "March" ]
  • toString():将数组转换为字符串,并返回结果
  const array1 = [1, 2, "a", "1a"]; 

  console.log(array1.toString()); 
  // -> 1,2,a,1a

JavaScript数据类型备忘单

var age = 18; // Number

var name = "Rahul"; // string

var name = {first:"Rahul", last:"Singh"}; // object

var truth = false; // boolean

var sheets = ["HTML", "CSS", "JS"]; // array

var a; typeof a; // undefined 

var a = null; // value null

JavaScript运算符备忘单

a = b + c - d; // addition, substraction

a = b * (c / d); // multiplication, division

x = 100 % 48; // modulo. 100 / 48 remainder = 4

a++; b--; // postfix increment and decrement

变量备忘单

  • var:最常见的变量。可以重新分配,但只能在函数中访问。执行代码时,用var定义的变量移到顶部。
  • const:在出现在代码中之前无法重新分配并且无法访问
  • let:与const类似,但是可以重新分配let变量,但不能重新声明
var a;            // variable
var b = "init";   // string
var c = "Hi" + " + "Rahul"; // "Hi Rahul"
var d = 1 + 2 + "3";   // "33"
var e = [2,3,5,8];   // array
var f = false;       // boolean
var g = /()/; // RegEx
var h = function(){};   // function object
const PI = 3.14;        // constant
var a = 1, b = 2, c = a + b;    // one line
let z = 'zzz';        // block scope local variable

获取日期方法提示表

  • getFullYear():根据当地时间返回指定日期的年份
  const moonLanding = new Date("January 08, 69 00:20:10"); 

  console.log(moonLanding.getFullYear()); 
  // -> 1969

  • getMonth():根据本地时间返回指定日期中的月份,该值从零开始(其中零表示一年的第一个月)。
  const moonLanding = new Date("January 08, 69 00:20:10"); 

  console.log(moonLanding.getMonth()); // (January gives 0)
  // -> 6
  • getDate():根据当地时间返回指定日期的月份
  const birthday = new Date("June 16, 2004 23:14:00"); 
  const date1 = birthday.getDate(); 

  console.log(date1); 
  // -> 19
  • getHours():根据当地时间返回指定日期的小时
  const birthday = new Date("June 16, 04 4:20"); 

  console.log(birthday.getHours()); 
  // -> 4
  • getMinutes():根据当地时间返回指定日期的分钟
  const birthday = new Date("June 16, 04 04:10"); 

  console.log(birthday.getMinutes());
  // -> 20
  • getSeconds()根据当地时间返回指定日期中的秒数
  const moonLanding = newDate("June 16, 69 00:23:11"); 

  console.log(moonLanding.getSeconds()); 
  // -> 18
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