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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsJavaScript hoisting, objects, this, and new are related, but they are not one shared “context” rule. To predict what code does, track when a binding is initialized, where a property is found, how a function is called, and whether construction creates a new receiver.
How JavaScript objects and property lookup work
An object is a collection of properties whose keys are strings or symbols. A property’s value can be data, another object, or a function. Use dot notation when the property name is a valid identifier, or bracket notation when it is computed or otherwise needs quoting:
const user = {
name: "Mina",
"favorite color": "blue"
};
user.name; // "Mina"
user["favorite color"]; // "blue"
user.name = "Ravi";
A function stored in an object property is commonly called a method. That label describes how the function is being used; it does not permanently bind the function to that object.
When JavaScript looks up a property, it first checks the object itself. If the property is not there, lookup can continue through the object’s prototype chain. An object’s own property can therefore shadow an inherited property with the same name. Not every object inherits from Object.prototype; an object can instead have a null prototype. See MDN’s guide to working with objects for more property examples.
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Own properties and shared prototype properties
Properties placed on a prototype can be shared through lookup by instances linked to that prototype. They do not become own properties on each instance. For example, if a constructor’s prototype supplies a start method, instances can use it without each storing a separate start property. Assigning car1.color, by contrast, creates or updates a property on car1; it does not set the color on other instances. An own property can also shadow a prototype property for that instance.
What hoisting means—and what it does not mean
Hoisting is a shorthand for observable declaration behavior, not a claim that JavaScript physically moves source lines before running them. MDN notes that “Hoisting is not a term normatively defined in the ECMAScript specification.” A more useful way to reason about it is to ask whether a binding exists at a point in the code, whether it has been initialized, and what value is available there. The answer depends on the declaration type and scope. See MDN’s hoisting explanation.
var is available as undefined before its assignment
A var binding is initialized to undefined before the assignment runs. Reading it earlier does not read the later assigned value:
console.log(a); // undefined
var a = 1;
console.log(a); // 1
This behavior does not move the assignment. The first log runs before a = 1.
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let and const have a temporal dead zone
Lexical declarations such as let, const, and class are associated with their scope before their declaration executes, but they cannot be accessed until initialized. Accessing one earlier throws a ReferenceError; the interval is called the temporal dead zone.
console.log(b); // ReferenceError
let b = 1;
This is a separate example: an uncaught exception stops execution at that line, so do not place it after the var example if you expect the rest of the same script to run.
Function declarations and imports
Function declarations can be called earlier in their scope than their textual position. Imports also have early availability semantics. These facts do not make all declarations interchangeable: var, lexical declarations, functions, and imports have different initialization and scope rules.
How this is chosen for a regular function
MDN’s concise rule is: “The value of this in JavaScript depends on how a function is invoked (runtime binding), not how it is defined.” For a regular function, inspect the call expression rather than where the function was written or stored. See MDN’s this reference.
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A method call supplies its receiver
In obj.method(), the object immediately to the left of the call supplies the receiver, so the method’s this is obj. Moving the same function into a standalone variable changes the call form:
"use strict";
const first = { value: 10, read() { return this.value; } };
const second = { value: 20 };
first.read(); // 10
second.read = first.read;
second.read(); // 20
const read = first.read;
read(); // TypeError when it tries to read this.value
In strict mode, the standalone call has this === undefined; the error above occurs because the function then tries to read value from undefined. In non-strict mode, a standalone regular-function call with an undefined or null receiver substitutes globalThis instead. The function’s result then depends on the properties and environment involved.
Explicitly set a regular function’s receiver
call and apply invoke a function with a chosen receiver; apply takes arguments as an array-like value. bind returns a new function whose receiver is fixed:
function readValue() { return this.value; }
const item = { value: 7 };
readValue.call(item); // 7
readValue.apply(item, []); // 7
const readItem = readValue.bind(item);
readItem(); // 7
Passing a callback does not by itself preserve the receiver from where it was defined. The API that invokes the callback determines the call behavior, so check that API’s contract when its this value matters.
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Arrow functions capture the surrounding this
An arrow function does not create its own this; it uses the value from its surrounding lexical scope. This makes arrows useful for callbacks that should retain an enclosing method’s receiver:
const counter = {
count: 0,
start() {
const tick = () => { this.count += 1; };
tick();
return this.count;
}
};
counter.start(); // 1
Because an arrow’s this is lexical, call, apply, and bind cannot replace it. Arrows are usually a poor choice for an object method when the intention is for the call site to select the receiver. An object literal also does not create a new this scope.
Top-level this depends on the environment
Do not infer a universal top-level value. In a browser classic script, top-level this is generally globalThis (the browser’s window). At the top level of a JavaScript module it is undefined. Node.js CommonJS wraps module code, so its top-level value differs from both cases.
What the new operator does
new Constructor() creates an object and calls the constructor with that object as this. In practical terms, the process is:
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- Create a fresh object.
- If the constructor’s
prototypeis an object, link the fresh object to it for property lookup. - Call the constructor with the fresh object as
this. - Return the constructor’s explicit non-primitive object result, if it returned one; otherwise return the fresh object.
For example, an instance can find describe through Car.prototype, while color is stored directly on that instance:
function Car(color) {
this.color = color;
}
Car.prototype.describe = function () {
return `A ${this.color} car`;
};
const car1 = new Car("red");
car1.describe(); // "A red car"
The prototype link supports inherited lookup; it does not copy prototype properties onto the instance. MDN documents the operator’s behavior and constructor examples in its new operator reference.
Constructor return values can change the result
If a constructor explicitly returns a different non-primitive object, that object becomes the result of the new expression instead of the fresh instance. A primitive return value, such as a string or number, is ignored for this purpose, and the fresh object is returned.
Quick Recap
Classes and constructor edge cases
- A class constructor must be called with
new; calling a class as an ordinary function throwsTypeError. - An ordinary constructor function can be called without
new, but that does not create an instance. Itsthisthen follows ordinary function-call rules. new.targetcan be used inside a function or constructor to detect whether it was invoked withnew.- Not every callable value is constructible. Arrow functions are not constructors, and some built-in functions are callable only or constructible only.
A practical way to predict JavaScript behavior
- For a variable read: identify its declaration and scope, then ask whether it has been initialized at that line. Distinguish
var’s initialundefinedfrom the temporal dead zone for lexical declarations. - For a property read: check the object’s own properties first, then follow its prototype chain until the property is found or lookup ends.
- For
thisin a regular function: inspect how the function is called. Check for a method receiver, an explicitcall/apply/bind, or a standalone call and its strictness. - For
thisin an arrow: look outward to the lexical scope where the arrow was created; the call site cannot replace that binding. - For
new: identify the constructor’s prototype, the receiver used during the call, and whether the constructor returns another object.
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