11道JS选择题

坊间传闻，现在的前端面试者的简历都写精通JS，那就拿这些题考考，若不能全答对，就不要说精通了，这只是中高级前端的基础题而已。

JS选择题

1.请问JS中的基本数据类型有几种？（ ）

• A.5
• B.6
• C.7

2.下面代码的输出是什么？（ ）

        function sayHi() {
            console.log(name);
            console.log(age);
            var name = "TJH";
            let age = 24;
        }

• A: TJH 和 undefined
• B: TJH 和 ReferenceError
• C: ReferenceError 和 24
• D: undefined 和 ReferenceError

3.下面代码的输出是什么？（ ）

        for (var i = 0; i < 3; i++) {
            setTimeout(function () {
                console.log(i);
            }, 1);
        }
        for (let i = 0; i < 3; i++) {
            setTimeout(function () {
                console.log(i);
            }, 1);
        }

• A: 0 1 2 and 0 1 2
• B: 0 1 2 and 3 3 3
• C: 3 3 3 and 0 1 2

4.下面代码的输出是什么？（ ）

        let a = 666;
        let b = new Number(666);
        let c = 666;
        console.log(a == b);
        console.log(a === b);
        console.log(b === c);

• A: true false true
• B: false false true
• C: true false false
• D: false true true

5.下面代码的输出是什么？（ ）　　

        const a = {};
        const b = { key: "b" };
        const c = { key: "c" };
        a[b] = 123;
        a[c] = 456;
        console.log(a[b]);

• A: 123
• B: 456
• C: undefined
• D: ReferenceError

6.下面代码的输出是什么？（ ）

        const numbers = [1, 2, 3];
        numbers[10] = 11;
        console.log(numbers);

• A: [1, 2, 3, 7 x null, 11]
• B: [1, 2, 3, 11]
• C: [1, 2, 3, 7 x empty, 11]
• D: SyntaxError

7.下面代码的输出是什么？（ ）

        let number = 0;
        console.log(number++);
        console.log(++number);
        console.log(number);

• A: 1 1 2
• B: 1 2 2
• C: 0 2 2
• D: 0 1 2

8.下面代码的输出是什么？（ ）

        let obj1 = {
            name: 'obj1_name',
            print: function () {
                return () => console.log(this.name);
            }
        }
        let obj2 = { name: 'obj2_name' };
        obj1.print()();
        obj1.print().call(obj2);
        obj1.print.call(obj2)();

• A: obj1_name obj2_name obj2_name
• B: obj2_name obj1_name obj2_name
• C: obj1_name obj1_name obj2_name

9.下面代码的输出是什么？（ ）

        const obj = { 1: "a", 2: "b", 3: "c" };
        const set = new Set([1, 2, 3, 4, 5]);
        obj.hasOwnProperty("1");
        obj.hasOwnProperty(1);
        set.has("1");
        set.has(1);

• A: false true false true
• B: false true true true
• C: true true false true
• D: true true true true

10.下面代码的输出是什么？（ ）

        function Foo() {
            getName = function () {
                console.log(1);
            };
            return this;
        }
        Foo.getName = function () {
            console.log(2);
        }
        Foo.prototype.getName = function () {
            console.log(3);
        };
        var getName = function () {
            console.log(4);
        };
        function getName() {
            console.log(5);
        }
        Foo.getName();
        getName();
        Foo().getName();
        getName();
        new Foo.getName();
        new Foo().getName();
        new new Foo().getName();

• A.4 2 1 1 2 3 3
• B.2 1 4 1 2 3 3
• C.2 4 1 1 3 2 3
• D.2 4 1 1 2 3 3

11.下面代码的输出是什么？（ ）

        async function async1() {
            console.log('async1 start');
            await async2();
            console.log('async1 end');
        }
        async function async2() {
            console.log('async2');
        }
        console.log('script start');
        setTimeout(function () {
            console.log('setTimeout0');
        }, 0);
        setTimeout(function () {
            console.log('setTimeout3');
        }, 0);
        setImmediate(() => console.log('setImmediate'));
        process.nextTick(() => console.log('nextTick'));
        async1();
        new Promise(function (resolve) {
            console.log('promise1');
            resolve();
            console.log('promise2');
        }).then(function () {
            console.log('promise3');
            });
        console.log('script end');

• A: script start - async2 start - async1 - promise1 - promise2 - script end - nextTick - async1 end - promise3 - setTimeout0 - setImmediate - setTimeout3
• B: script start - async1 start - async2 - promise2 - promise1 - script end - nextTick - async1 end - promise3 - setTimeout0 - setImmediate - setTimeout3
• C: script start - async1 start - async2 - promise1 - promise2 - script end - nextTick - async1 end - promise3 - setTimeout3 - setImmeidate - setTimeout0
• D: script start - async1 start - async2 - promise1 - promise2 - script end - nextTick - async1 end - promise3 - setTimeout0- setImmeidate - setTimeout3

　　

参考答案

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