This is the 16th day of my participation in the August More Text Challenge

Let’s do it. Let’s do it

The continue problem

for (let i = 1; i < 5; i++) {
  if (i === 3) continue;
  console.log(i);
}
Copy the code
  • A: 1 2
  • B: 1 2 3
  • C: 1 2 4
  • D: 1 3 4

Answer: C

In this for loop, continue skips the iteration when I is equal to 3.

Constructor problem

String.prototype.giveMyFood = () = > {
  return "Please give weirui food!";
};

const name = "weirui";

name.giveMyFood();
Copy the code
  • A: "Please give weirui food!"
  • B: TypeError: not a function
  • C: SyntaxError
  • D: undefined

Answer: A,

String is a built-in constructor to which we can add attributes. Add a method to its prototype here. A string of primitive type is automatically converted to a string object, which is generated by the string prototype function. Therefore, all strings (string objects) have access to this method!

When giveMyFood is called with a string of primitive type, the following actually happens:

  • To create aStringIs an instance of a wrapper type
  • Called on an instancesubstringmethods
  • Destroy instance

Type conversion

const a = {};
const b = { key: "b" };
const c = { key: "c" };

a[b] = 123;
a[c] = 456;

console.log(a[b]);
Copy the code
  • A: 123
  • B: 456
  • C: undefined
  • D: ReferenceError

A [“Object Object “] = 123 when the Object is automatically converted to a string, it becomes [Object Object]. C objects are also implicitly cast. So, a[“Object Object “] = 456. At this point we print a[b], which is actually a[“Object Object “], so return 456.

Call the bind problem

const person = { name: "weirui" };

function sayHi(age) {
  console.log(`The ${this.name} is ${age}`);
}

sayHi.call(person, 24);
sayHi.bind(person, 24);
Copy the code
  • A: undefined is 21 Lydia is 21
  • B: function function
  • C: Lydia is 21 Lydia is 21
  • D: Lydia is 21 function

Answer: D

We pass the object referenced by this keyword, but the.call method executes immediately,

The.bind method returns a copy of the value of the function, but with the context of the binding, it is not executed immediately.

Type judgment

function sayHi() {
  return (() = > 0) (); }typeof sayHi();
Copy the code
  • A: "object"
  • B: "number"
  • C: "function"
  • D: "undefined"

Answer: B

The sayHi function returns the return value of the function immediately called. This function returns 0 and is of type number.

JavaScript has seven built-in types: NULL, undefined, Boolean, number, String, Object, and symbol. Function is not a type because a function is an object and its type is Object.

Another type judgment problem:

Which values are false?

0;
new Number(0);
("");
("");
new Boolean(false);
undefined;
Copy the code
  • A: 0.' '.undefined
  • B: 0.new Number(0).' '.new Boolean(false).undefined
  • C: 0.' '.new Boolean(false).undefined
  • D: All are false values

Answer: A,

There are only six false values in JavaScript:

  • undefined
  • null
  • NaN
  • 0
  • ' ' (empty string)
  • false

Function constructors such as new Number and New Boolean are true values.