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  • js反轉數組的方法(前端面試題2021及答案)


    備忘單是我們開發人員始終需要的參考。因此,這里我編譯了許多JavaScript參考代碼。查看分類并找到它。這篇文章對學習者和開發人員都有幫助。

    終極JavaScript秘籍,建議收藏食用

    JavaScript Number方法備忘單

    • toExponential():以字符串形式返回表示Number對象的字符串
      function expo(x, f) {
          return 
          Number.parseFloat(x).toExponential(f);
      }
    
      console.log(expo(123456, 2)); 
      // -> 1.23e+5
    
    • toFixed():使用定點表示法格式化數字
      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對象的字符串
      function precise(x) {
          return
          Number.parseFloat(x).toPrecision(4); 
      }
    
      console.log(precise(123.456)); 
      // -> 123.5
    
    • toString():返回表示指定Number對象的字符串
      function hexColour(c) {
          if (c < 256) {
              return Math.abs(c).toString(16); 
          }
          return 0; 
      }
    
      console.log(hexColour(233)); 
      // -> e9
    
    • valueOf():返回數字對象的包裝原始值
      const numObj = new Number(42); 
      console.log(typeof numObj); 
      // -> object
    
      const num = numObj.valueOf(); 
      console.log(num); 
      // -> 42
    
      console.log(typeof num); 
      // -> number
    
    

    JavaScript循環備忘單

    • 對于循環
      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循環
      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, 
    
    • 循環執行
      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, 
    
    • 繼續
      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字符串方法備忘單

    • charAt():返回指定索引處的字符
      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():連接兩個或多個字符串,并返回所連接字符串的副本
      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(或正則表達式)和字符串之間的匹配項,并將匹配的子字符串替換為新的子字符串
      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():搜索正則表達式和字符串之間的匹配項,并返回匹配項的位置
      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():刪除字符串兩端的空格
      const greeting = "  Hello world!   "; 
    
      console.log(greeting); 
      // -> Hello world!
    
      console.log(greeting.trim()); 
      // -> Hello world!
    
    • substr():從字符串中提取字符,從指定的起始位置開始,直到指定的字符數
      const str = "Mozilla"; 
    
      console.log(str.substr(1, 2)); 
      // -> oz
    
      console.log(stre.substr(2)); 
      // -> zilla
    
    • toLowerCase():將字符串轉換為小寫字母
      const sentence = "Elon became the richest last night."; 
    
      console.log(sentence.toLowerCase()); 
      // -> elon became the richest last night.
    

    JavaScript數組方法指導表

    • concat():連接兩個或多個數組,并返回聯接數組的副本
      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():在數組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():將數組的所有元素連接到一個字符串中
      let elements = ["Fire", "Air", "Water"]; 
    
      console.log(elements.join()); 
      // -> Fire,Air,Water
    
      console.log(elements.join(" ")); 
      // -> Fire Air Water
    
    • pop():刪除數組的最后一個元素,并返回該元素
      let plants = ["broccoli", "cauliflower", "cabbage", "kale", "tomato"]; 
    
      console.log(plants.pop()); 
      // -> tomato
    
      console.log(plants); 
      // -> Array(4) ["brocxoli", "cauliflower", "cabbage", "kale"]
    
    • reverse():反轉數組中元素的順序
      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():刪除數組的第一個元素,并返回該元素
      let array1 = [1, 2, 3]; 
    
      let firstElement = array1.shift(); 
    
      console.log(array1); 
      // -> Array [ 2, 3 ]
    
    • sort():對數組的元素進行排序
      let months = ["March", "Jan", "Feb", "Dec"]; 
      months.sort(); 
    
      console.log(months); 
      // -> Array(4) [ "Dec", "Feb", "Jan", "March" ]
    
    • toString():將數組轉換為字符串,并返回結果
      const array1 = [1, 2, "a", "1a"]; 
    
      console.log(array1.toString()); 
      // -> 1,2,a,1a
    

    JavaScript數據類型備忘單

    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運算符備忘單

    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
    

    變量備忘單

    • var:最常見的變量??梢灾匦路峙?,但只能在函數中訪問。執行代碼時,用var定義的變量移到頂部。
    • const:在出現在代碼中之前無法重新分配并且無法訪問
    • let:與const類似,但是可以重新分配let變量,但不能重新聲明
    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
    

    獲取日期方法提示表

    • getFullYear():根據當地時間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():根據本地時間返回指定日期中的月份,該值從零開始(其中零表示一年的第一個月)。
      const moonLanding = new Date("January 08, 69 00:20:10"); 
    
      console.log(moonLanding.getMonth()); // (January gives 0)
      // -> 6
    
    • getDate():根據當地時間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():根據當地時間返回指定日期的小時
      const birthday = new Date("June 16, 04 4:20"); 
    
      console.log(birthday.getHours()); 
      // -> 4
    
    • getMinutes():根據當地時間返回指定日期的分鐘
      const birthday = new Date("June 16, 04 04:10"); 
    
      console.log(birthday.getMinutes());
      // -> 20
    
    • getSeconds()根據當地時間返回指定日期中的秒數
      const moonLanding = newDate("June 16, 69 00:23:11"); 
    
      console.log(moonLanding.getSeconds()); 
      // -> 18

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