---

name: javascript-mastery description: "每个开发者都应该掌握的 33+ 个核心 JavaScript 概念，灵感来自 33-js-concepts。" risk: unknown source: community date_added: "2026-02-27"

JavaScript 精通指南

每个开发者都应该掌握的 33+ 个核心 JavaScript 概念，灵感来自 33-js-concepts。

何时使用本技能

适用于以下场景：

• 解释 JavaScript 概念
• 调试棘手的 JS 行为
• 教授 JavaScript 基础知识
• 审查代码中的 JS 最佳实践
• 理解语言特性

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1. 基础知识

1.1 原始类型

JavaScript 有 7 种原始类型：

// String
const str = "hello";

// Number (integers and floats)
const num = 42;
const float = 3.14;

// BigInt (for large integers)
const big = 9007199254740991n;

// Boolean
const bool = true;

// Undefined
let undef; // undefined

// Null
const empty = null;

// Symbol (unique identifiers)
const sym = Symbol("description");

要点：

• 原始类型是不可变的
• 按值传递
• typeof null === "object" 是一个历史遗留 bug

1.2 类型强制转换

JavaScript 会隐式转换类型：

// String coercion
"5" + 3; // "53" (number → string)
"5" - 3; // 2    (string → number)

// Boolean coercion
Boolean(""); // false
Boolean("hello"); // true
Boolean(0); // false
Boolean([]); // true (!)

// Equality coercion
"5" == 5; // true  (coerces)
"5" === 5; // false (strict)

假值（共 8 个）： false、0、-0、0n、""、null、undefined、NaN

1.3 相等运算符

// == (loose equality) - coerces types
null == undefined; // true
"1" == 1; // true

// === (strict equality) - no coercion
null === undefined; // false
"1" === 1; // false

// Object.is() - handles edge cases
Object.is(NaN, NaN); // true (NaN === NaN is false!)
Object.is(-0, 0); // false (0 === -0 is true!)

规则：始终使用 ===，除非有特定理由不这样做。

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2. 作用域与闭包

2.1 作用域类型

// Global scope
var globalVar = "global";

function outer() {
  // Function scope
  var functionVar = "function";

  if (true) {
    // Block scope (let/const only)
    let blockVar = "block";
    const alsoBlock = "block";
    var notBlock = "function"; // var ignores blocks!
  }
}

2.2 闭包

闭包是一个能记住其词法作用域的函数：

function createCounter() {
  let count = 0; // "closed over" variable

  return {
    increment() {
      return ++count;
    },
    decrement() {
      return --count;
    },
    getCount() {
      return count;
    },
  };
}

const counter = createCounter();
counter.increment(); // 1
counter.increment(); // 2
counter.getCount(); // 2

常见用例：

• 数据私有化（模块模式）
• 函数工厂
• 偏函数应用
• 记忆化

2.3 var vs let vs const

// var - function scoped, hoisted, can redeclare
var x = 1;
var x = 2; // OK

// let - block scoped, hoisted (TDZ), no redeclare
let y = 1;
// let y = 2; // Error!

// const - like let, but can't reassign
const z = 1;
// z = 2; // Error!

// BUT: const objects are mutable
const obj = { a: 1 };
obj.a = 2; // OK
obj.b = 3; // OK

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3. 函数与执行

3.1 调用栈

function first() {
  console.log("first start");
  second();
  console.log("first end");
}

function second() {
  console.log("second");
}

first();
// Output:
// "first start"
// "second"
// "first end"

栈溢出示例：

function infinite() {
  infinite(); // No base case!
}
infinite(); // RangeError: Maximum call stack size exceeded

3.2 变量提升

// Variable hoisting
console.log(a); // undefined (hoisted, not initialized)
var a = 5;

console.log(b); // ReferenceError (TDZ)
let b = 5;

// Function hoisting
sayHi(); // Works!
function sayHi() {
  console.log("Hi!");
}

// Function expressions don't hoist
sayBye(); // TypeError
var sayBye = function () {
  console.log("Bye!");
};

3.3 this 关键字

// Global context
console.log(this); // window (browser) or global (Node)

// Object method
const obj = {
  name: "Alice",
  greet() {
    console.log(this.name); // "Alice"
  },
};

// Arrow functions (lexical this)
const obj2 = {
  name: "Bob",
  greet: () => {
    console.log(this.name); // undefined (inherits outer this)
  },
};

// Explicit binding
function greet() {
  console.log(this.name);
}
greet.call({ name: "Charlie" }); // "Charlie"
greet.apply({ name: "Diana" }); // "Diana"
const bound = greet.bind({ name: "Eve" });
bound(); // "Eve"

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4. 事件循环与异步

4.1 事件循环

console.log("1");

setTimeout(() => console.log("2"), 0);

Promise.resolve().then(() => console.log("3"));

console.log("4");

// Output: 1, 4, 3, 2
// Why? Microtasks (Promises) run before macrotasks (setTimeout)