---
id: javascript-strict-mode
title: "JavaScript Strict Mode"
category: "Frontend"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["use strict", "strict mode", "ECMAScript 5 strict", "strict directive", "JS strict"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.88
created_at: 2026-06-23
updated_at: 2026-06-23
review_reason: ""
merge_history: []
tags: ["javascript", "js", "web", "frontend", "w3schools", "strict-mode", "use-strict", "es5"]
raw_sources: ["https://www.w3schools.com/js/js_strict.asp"]
applied_in: []
github_commit: ""
---

# [[JavaScript Strict Mode]]

## 🎯 한 줄 통찰 (One-line insight)
`"use strict";` (ECMAScript 5) makes JavaScript run in strict mode, turning previously-tolerated "bad syntax" into real errors. [S1]

## 🧠 핵심 개념 (Core concepts)
- **`"use strict"` is a directive** — it is a literal expression, ignored by older JavaScript versions, that tells the engine to execute code in strict mode. [S1]
- **Introduced in ES5** — strict mode was added in ECMAScript 5. [S1]
- **Two scopes** — declared at the top of a script it applies globally; declared at the top of a function it applies only inside that function. [S1]
- **Must appear at the beginning** — the directive must be at the start of the script or function to be recognized. [S1]
- **Turns mistakes into errors** — strict mode makes it easier to write secure code by converting silent bad syntax into thrown errors (e.g. prevents accidental globals). [S1]

## 🧩 추출된 패턴 (Extracted patterns)
- **Opt into safety per file or per function** — place `"use strict";` at the top of a script (global) or a function (local). [S1]
- **Declare before use** — strict mode forbids using undeclared variables/objects, forcing explicit declarations. [S1]
- **Avoid deprecated/unsafe constructs** — `with`, octal literals, duplicate parameters, deleting variables, etc. are all disallowed. [S1]

## 📖 세부 내용 (Details)
**Declaring strict mode**
Strict mode is declared by adding `"use strict";` to the beginning of a script or a function. Declared at the beginning of a script, it has global scope (all code in the script executes in strict mode): [S1]
```javascript
"use strict";
x = 3.14;       // This will cause an error because x is not declared
```
Declared inside a function, it has local scope (only the code inside the function is in strict mode): [S1]
```javascript
"use strict";
myFunction();

function myFunction() {
  y = 3.14;   // This will cause an error
}
```

**Why strict mode?**
Strict mode makes it easier to write "secure" JavaScript. It changes previously accepted "bad syntax" into real errors. For example, in normal JavaScript, mistyping a variable name creates a new global variable; in strict mode this throws an error, so you cannot accidentally create a global variable. [S1]

**Not allowed in strict mode**

Using a variable (or object) without declaring it: [S1]
```javascript
"use strict";
x = 3.14;          // This will cause an error
```
```javascript
"use strict";
x = {p1:10, p2:20};   // This will cause an error
```

Deleting a variable (or object) or a function is not allowed: [S1]
```javascript
"use strict";
let x = 3.14;
delete x;            // This will cause an error
```
```javascript
"use strict";
function x(p1, p2) {};
delete x;            // This will cause an error
```

Duplicating a parameter name is not allowed: [S1]
```javascript
"use strict";
function x(p1, p1) {};   // This will cause an error
```

Octal numeric literals and octal escape characters are not allowed: [S1]
```javascript
"use strict";
let x = 010;        // This will cause an error
```
```javascript
"use strict";
let x = "\010";     // This will cause an error
```

Writing to a read-only property is not allowed: [S1]
```javascript
"use strict";
const obj = {};
Object.defineProperty(obj, "x", {value:0, writable:false});
obj.x = 3.14;       // This will cause an error
```

Writing to a get-only property is not allowed: [S1]
```javascript
"use strict";
const obj = {get x() {return 0} };
obj.x = 3.14;       // This will cause an error
```

Deleting an undeletable property is not allowed: [S1]
```javascript
"use strict";
delete Object.prototype;   // This will cause an error
```

The word `eval` cannot be used as a variable: [S1]
```javascript
"use strict";
let eval = 3.14;    // This will cause an error
```

The word `arguments` cannot be used as a variable: [S1]
```javascript
"use strict";
let arguments = 3.14;   // This will cause an error
```

The `with` statement is not allowed: [S1]
```javascript
"use strict";
with (Math){x = cos(2)};   // This will cause an error
```

For security reasons, `eval()` is not allowed to create variables in the scope from which it was called: [S1]
```javascript
"use strict";
eval ("var x = 2");
alert (x);          // This will cause an error
```

**The "use strict" directive**
The `"use strict"` directive is only recognized at the beginning of a script or a function. [S1]

## 🛠️ 적용 사례 (Applied in summary)
The page's snippets are the applied cases: each shows a construct that runs silently (or wrongly) in normal mode but throws under `"use strict";` — undeclared assignment, `delete` on variables/functions, duplicate parameters, octal literals, read-only/get-only writes, reserved words `eval`/`arguments`, `with`, and `eval()` scope injection. No external project/commit applications found in the source.

## 💻 코드 패턴 (Code patterns)
Enable strict mode globally (top of script) (language: JavaScript):
```javascript
"use strict";
x = 3.14;   // Error: x is not declared
```
Enable strict mode for a single function:
```javascript
function myFunction() {
  "use strict";
  // strict-mode code here
}
```

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
No contradictions found in the source. The page notes that older browsers/JavaScript versions simply ignore the `"use strict";` string, so it is backward-compatible.

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual (실제 적용 사례 발견 시 applied/validated로 승격 가능)
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.88
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[JavaScript Tutorial]]
- **관련 개념:** [[JavaScript Scope]], [[JavaScript Hoisting]], [[JavaScript var let const]], [[JavaScript Best Practices]]
- **참조 맥락:** Referenced when hardening JavaScript against silent errors and accidental globals.

## 📚 출처 (Sources)
- [S1] W3Schools — JavaScript Strict Mode — https://www.w3schools.com/js/js_strict.asp

## 📝 변경 이력 (Change history)
- 2026-06-23: Initial draft synthesized from the W3Schools "JavaScript Strict Mode" page (Astra wiki-curation, P-Reinforce v3.1 format).