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Javascript学习笔记

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KaiWn

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1.箭头函数

箭头函数是一种更短的函数表达式。

const age = birthyear => 2022 - birthyear; 
console.log(age(2000))

箭头左边的birthyear是参数,箭头右边是要执行的代码块。在编写如上单行函数时,我们不需要写花括号,也不需要写return关键字,但实际上这些都是隐式发生的。

  • 多行函数的情况:使用花括号 ' { } '
const years = birthyear => {
    const age = 2022 - birthyear;
    const retirement = 65 - age;
    return retirement;    
}
console.log(years(2000));
  • 也可以使用多个参数:(brithyear,name)
const your_age = (brithyear,name) => {
    const age = 2022 - brithyear;
    return `${name},you are ${age} years old `;
} 
console.log(your_age(2000,'soria'));

2. 数组

  • 构造一个数组

    const friends = ['mike','adams','pat'];
    console.log(friends);
  • 使用Arroy函数构造数组

    const years = new Array(1991,1984,2008,2020);
    console.log(years);

    注意Array的 ' A ' 要大写,array前面要加上new关键字。

  • 查看数组的长度

    const friends = ['mike','adams','pat'];
    console.log(friends.length);
  • 数组的引索

    const friends = ['mike','adams','pat'];
    console.log(friends[0],friends[1],friends[2],friends[3]);
  • 更改数组里的元素

    const friends = ['mike','adams','pat'];
    friends[2] = 'jay';
    console.log(friends);
  • 数组的运算

    console.log(2037 - [1990,1967]); 
    console.log(Number([1990,1967]));// 数组强制转换为数字类型 结果为NaN
    console.log(2037 + [1990,1967]); // 数组强制转换为字符串

3. 数组的方法

  • push函数

    const friends = ['mike','adams','pat'];
    const newlength = friends.push('jay'); 
    //push在数组末尾添加值'jay' , 同时可以返回新数组的长度
    console.log(friends);
    console.log(newlength);
  • unshift函数

    const friends = ['mike','adams','pat'];
    const newlength = friends.unshift('jay'); 
    //unshift在数组开头添加值'jay' , 同时可以返回新数组的长度
    console.log(friends);
    console.log(newlength);
  • pop函数

    const friends = ['mike','adams','pat'];
    friends.pop();
    console.log(friends);
    const popped = friends.pop();// 删除数组末尾的值'adams',同时返回这个值
    console.log(popped);
    console.log(friends);
  • include函数

    const friends = ['mike','adams','pat'];
    console.log(friends.includes('mike'))
    // 查找数组里是否含有'mike',返回一个bool值
    
    if (friends.includes('mike')){
        console.log('you have a friend called mike')
    }
  • indexOf函数

    const friends = ['mike','adams','pat'];
    console.log(friends.indexOf('mike'))
    // 返回'mike'的引索

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