了解 JavaScript 中的变量、作用域和起重功能

1// Assign the string value Sammy to the username identifier
2var username = "sammy_shark";

1// Check if variable is equal to value
2if (username === "sammy_shark") {
3  console.log(true);
4}

1[secondary_label Output]
2true

1// Assignment of various variables
2var name = "Sammy";
3var spartans = 300;
4var kingdoms = [ "mammals", "birds", "fish" ];
5var poem = { roses: "red", violets: "blue" }; 
6var success = true;
7var nothing = null;

1// Send spartans variable to the console
2console.log(spartans);

1[secondary_label Output]
2300

1// Assign value to password variable
2var password = "hunter2";
3
4// Reassign variable value with a new value
5password = "hunter3";
6
7console.log(password);

1[secondary_label Output]
2'hunter3'

1// Initialize a global variable
2var creature = "wolf";

 1// Initialize a global variable
 2var species = "human";
 3
 4function transform() {
 5  // Initialize a local, function-scoped variable
 6  var species = "werewolf";
 7  console.log(species);
 8}
 9
10// Log the global and local variable
11console.log(species);
12transform();
13console.log(species);

1[secondary_label Output]
2human
3werewolf
4human

 1var fullMoon = true;
 2
 3// Initialize a global variable
 4let species = "human";
 5
 6if (fullMoon) {
 7  // Initialize a block-scoped variable
 8  let species = "werewolf";
 9  console.log(`It is a full moon. Lupin is currently a ${species}.`);
10}
11
12console.log(`It is not a full moon. Lupin is currently a ${species}.`);

1[secondary_label Output]
2It is a full moon. Lupin is currently a werewolf.
3It is not a full moon. Lupin is currently a human.

 1// Use var to initialize a variable
 2var species = "human";
 3
 4if (fullMoon) {
 5  // Attempt to create a new variable in a block
 6  var species = "werewolf";
 7  console.log(`It is a full moon. Lupin is currently a ${species}.`);
 8}
 9
10console.log(`It is not a full moon. Lupin is currently a ${species}.`);

1[secondary_label Output]
2It is a full moon. Lupin is currently a werewolf.
3It is not a full moon. Lupin is currently a werewolf.

1// Attempt to use a variable before declaring it
2console.log(x);
3
4// Variable assignment
5var x = 100;

1[secondary_label Output]
2undefined

1// Attempt to use a variable before declaring it
2console.log(x);
3
4// Variable assignment without var
5x = 100;

1[secondary_label Output]
2ReferenceError: x is not defined

1// The code we wrote
2console.log(x);
3var x = 100;
4
5// How JavaScript interpreted it
6var x;
7console.log(x);
8x = 100;

 1// Initialize x in the global scope
 2var x = 100;
 3
 4function hoist() {
 5  // A condition that should not affect the outcome of the code
 6  if (false) {
 7    var x = 200;
 8  }
 9  console.log(x);
10}
11
12hoist();

1[secondary_label Output]
2undefined

 1// Initialize x in the global scope
 2let x = true;
 3
 4function hoist() {
 5  // Initialize x in the function scope
 6  if (3 === 4) {
 7    let x = false;
 8  }
 9  console.log(x);
10}
11
12hoist();

1
2[secondary_label 输出]
3真

1// Attempt to overwrite a variable declared with var
2var x = 1;
3var x = 2;
4
5console.log(x);

1[secondary_label Output]
22

1// Attempt to overwrite a variable declared with let
2let y = 1;
3let y = 2;
4
5console.log(y);

1[secondary_label Output]
2Uncaught SyntaxError: Identifier 'y' has already been declared

1// Assign value to const
2const SPECIES = "human"; 
3
4// Attempt to reassign value
5SPECIES = "werewolf";
6
7console.log(SPECIES);

1[secondary_label Output]
2Uncaught TypeError: Assignment to constant variable.

1// Declare but do not initialize a const
2const TODO;
3
4console.log(TODO);

1[secondary_label Output]
2Uncaught SyntaxError: Missing initializer in const declaration

 1// Create a CAR object with two properties
 2const CAR = {
 3    color: "blue",
 4    price: 15000
 5}
 6
 7// Modify a property of CAR
 8CAR.price = 20000;
 9
10console.log(CAR);

1[secondary_label Output]
2{ color: 'blue', price: 20000 }