Code output results of front-end interview (iii)

This is the 19th day of my participation in the August More Text Challenge.

Execute functions and scopes immediately

var a = 123;
(function a () {
    a = 'abc';
    console.log(a); }) ();Copy the code

The answer:

ƒ a () {a ='abc';
    console.log(a);
}
Copy the code

instructions

• Executing the function IIFE immediately simulates block-level scope, declaring variables inside a function expression and then calling the function immediately. Variables in the function body scope are as if they were in the block-level scope.
• If the immediate function name is the same as the internal variable name, the function name takes precedence
• In strict mode ‘use strict’; The following code will report an error

The String and the switch

var str = new String(123);
switch (str) {
  case '123': {
    console.log('string 123');
    break;
  } 
  case 123: {
    console.log(`number 123`);
    break;
  } 
  case undefined: {
    console.log(`undefined value`);
    break;
  } 
  default: {
    console.log('unknown value');
    break; }}Copy the code

Answer: Unknown value is displayed

instructions

• 1. var str = new String(123); STR is an example of the String class of type Object
• 2. Switch is strictly comparative, so STR === ‘123’ is false
• 3. STR does not match any of the preceding values. Run the default command to output unknown value
• 4. MDN mentions that a switch statement first evaluates its expression. It then starts from the first case clause until it finds a clause whose expression value is equal to the value of the input expression (usingStrict operators (EN-US).= = =) and transfers control to the clause to execute the statement.

Object. GetPrototypeOf prototype

function A() {};
var a0 = new A();
var a1= {};
a1.__proto__ = A.prototype;
var a2 = {};
a2.__proto__ = Object.getPrototypeOf(new A());
var a3 = {};
a3.__proto__ = Object.getPrototypeOf(A);

console.log(a0 instanceof A);
console.log(a1 instanceof A);
console.log(a2 instanceof A);
console.log(a3 instanceof A);
Copy the code

Answer :true, true, true, false

instructions

The object.getProtoTypeof () method returns the Prototype of the specified Object (the value of the internal [[Prototype]] property)

var a0 = new A();
// the a0 prototype __proto__ automatically points to a.prototype
// so a0 instanceof A is true;


var a1= {};
a1.__proto__ = A.prototype;
// a1 prototype __proto__ points directly to a.prototype
// so a0 instanceof A is true;


var a2 = {};
a2.__proto__ = Object.getPrototypeOf(new A());
// object.getProtoTypeof (new A()) is A prototype pointing to A new A() Object.
// So a2.__proto__ points to a.prototype
// so a2 instanceof A is true;


var a3 = {};
a3.__proto__ = Object.getPrototypeOf(A);
// object.getProtoTypeof (A) is A prototype pointing to A function.
// so a2.__proto__ points to function.prototype
// so a2 instanceof A is false;
Copy the code