Add strict mode question

blog_posts/javascript-use-strict.md

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+---
+title: What does 'use strict' do and what are some of the key benefits to using it?
+type: question
+tags: javascript,function,object
+authors: chalarangelo
+cover: blog_images/palm-tree-house.jpg
+excerpt: JavaScript's strict mode can make your code faster, cleaner and more secure.
+firstSeen: 2021-11-28T05:00:00-04:00
+---
+
+Strict mode can be applied to entire scripts or individual functions by including `'use strict'` before any other statements.
+
+```js
+// script.js
+'use strict';
+const x = "Hello from a strict mode script";
+
+// other.js
+function strict() {
+  'use strict';
+  const x = 'Hello from a strict mode function';
+}
+```
+
+This enforces more strict parsing and error handling of JavaScript code, as described below.
+
+### No accidental global variables
+
+Strict mode makes it impossible to accidentally create global variables due to mistyped variable names. Assignments, which would accidentally create global variables, instead throw an error in strict mode:
+
+```js
+'use strict';
+myVariable = 42;
+// The above line will throw a ReferenceError, assuming no global
+// variable named myVariable has been declared previously
+```
+
+### Elimination of silent errors
+
+Strict mode changes some previously-accepted mistakes into errors. These include:
+
+- Assignments which would otherwise silently fail
+- Deleting undeletable properties or plain names
+- Duplicated names in function arguments
+- 0-prefixed octal literals
+- Setting properties on primitives
+
+```js
+'use strict';
+
+let undefined = 5; // TypeError (non-writable global)
+let obj = {};
+Object.defineProperty(obj1, 'x', { value: 1, writable: false });
+obj.x = 2; // TypeError (non-writable property);
+
+delete Object.prototype; // TypeError (undeletable property)
+delete something; // SyntaxError (plain name)
+
+const sum (a, b, b) { // SyntaxError (duplicated argument name)
+  return a + b + b;
+}
+
+const x = 012; // SyntaxError (0-prefixed octal literal)
+
+false.true = 10; // TypeError (property on primitive)
+```
+
+### Simplified `eval`
+
+Strict mode makes `eval` more transparent by preventing the introduction of new variables in the surrounding scope. In strict mode, `eval` creates variables only for the code being evaluated.
+
+```js
+'use strict';
+let x = 1;
+eval('let x = 3; console.log(x);'); // LOGS: 3
+console.log(x); // LOGS: 1
+```
+
+### Simplified `arguments`
+
+Strict mode simplifies `arguments`, by removing some of their side effects. `arguments` aren't aliased, thus they always refer to the original arguments when the function was invoked. Moreover, `arguments.callee` and `arguments.caller` are no longer supported.
+
+```js
+'use strict';
+function f(x) {
+  x = 5;
+  return x === arguments[0];
+}
+
+f(10); // false
+```
+
+### No `this` boxing
+
+Strict mode makes JavaScript more secure, by restricting access the global object via `this`. In strict mode, `this` is not boxed (forced into being an object), meaning that if unspecified it will be `undefined` instead of the global object.
+
+```js
+'use strict';
+function f() {
+  return this;
+}
+
+f(); // undefined
+```
+
+### Other changes
+
+Strict mode implements a few more, less commonly-mentioned changes. These include:
+
+- `with` is prohibited, resulting in an error if used
+- `Function.prototype.arguments` and `Function.prototype.caller` are non-deletable properties which throw when set or retrieved
+- `implements`, `interface`, `let`, `package`, `private`, `protected`, `public`, `static`, and `yield` are reserved keywords
+- Function statements are only allowed at the top level of a script of function