JSE Certified Entry-Level JavaScript Programmer flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Hoisting

Declarations are processed before code runs: var is initialized to undefined, function declarations are fully available, and let/const exist but stay uninitialized in the temporal dead zone.

Automatic semicolon insertion

The parser inserts a semicolon at a line break only when the next token cannot continue the statement, which is why a line beginning with a parenthesis can merge with the previous line.

Strict mode

Enabled by the directive 'use strict', it makes undeclared assignments throw ReferenceError, forbids duplicate parameter names, and changes this to undefined in plain function calls.

Temporal dead zone

The span between entering a block and executing a let or const declaration, during which accessing the binding throws ReferenceError instead of returning undefined.

Falsy values

Exactly eight values coerce to false in a boolean context: false, 0, -0, 0n, the empty string, null, undefined, and NaN. Empty arrays and objects are truthy.

Loose equality coercion

The == operator converts operands to a common type before comparing: booleans become numbers, strings compared to numbers become numbers, and null equals only undefined.

Short-circuit evaluation

The && and || operators stop evaluating once the result is determined and return an operand rather than a boolean, so expressions with side effects may not run.

Nullish coalescing operator

The ?? operator returns its right operand only when the left side is null or undefined, unlike || which also falls back for 0, empty string, and false.

Ternary conditional operator

The only operator taking three operands: condition ? valueIfTrue : valueIfFalse. It associates right to left and is often used for inline branching.

Operator precedence

The fixed ranking that decides evaluation order, from member access and unary operators down through multiplication, addition, comparison, equality, logical operators, and assignment.

Switch fall-through

Cases are compared using strict equality and continue executing into later cases until a break, return, or throw exits the switch block.

for...of loop

Iterates the values produced by an iterable's iterator, working with arrays, strings, Map, and Set, but throwing TypeError on plain objects that lack Symbol.iterator.

for...in loop

Iterates enumerable property keys as strings, including inherited ones, so it is suited to object inspection and generally avoided for arrays.

Labeled break

A label before a loop lets break label or continue label target that specific outer loop from inside a nested loop, which plain break cannot do.

Closure

A function retains access to variables from the lexical scope where it was created, keeping those variables alive after the outer function has returned.

Arrow function this

Arrow functions have no own this; they capture this from the surrounding scope at definition time, so call, apply, and bind cannot change it.

Default parameters

A parameter default applies only when the argument is undefined, is evaluated at call time from left to right, and may reference earlier parameters.

IIFE

An immediately invoked function expression runs once at definition and creates a private scope; wrapping parentheses are required so it parses as an expression.

Array length property

Length equals the highest index plus one and is writable; setting it smaller truncates the array, and setting it larger creates empty slots.

splice versus slice

splice(start, count, ...items) mutates the array and returns removed elements; slice(start, end) returns a new shallow copy without changing the original.

Default array sort

sort converts elements to strings and compares UTF-16 code units, so numbers order lexicographically unless a comparator such as (a, b) => a - b is supplied.

Object destructuring

A binding pattern extracts properties by key, supports renaming with {a: x}, nested patterns, and defaults that apply when the value is undefined.

JSON.stringify omissions

Properties whose values are undefined, functions, or symbols are dropped from objects and become null inside arrays; circular references throw TypeError.

Map versus plain object

A Map accepts keys of any type, preserves insertion order, exposes size as a property, and provides get, set, has, and delete methods.

toFixed method

Number.prototype.toFixed(digits) returns a string rounded to the given decimal places, so the result must be converted back with Number or parseFloat for arithmetic.

finally block

Code in finally always runs after try and catch, even when try returns or throws, unless the process exits, making it the place for cleanup.

TypeError versus ReferenceError

ReferenceError means an identifier was never declared; TypeError means a value of the wrong type was used, such as calling a non-function or reading a property of undefined.

Optional chaining

The ?. operator short-circuits to undefined when the value before it is null or undefined, preventing TypeError when accessing nested properties that may be missing.

querySelectorAll static list

It returns a NodeList snapshot matching a CSS selector at call time; later DOM changes are not reflected, unlike live HTMLCollections from getElementsByClassName.

Event bubbling

After the target handles an event, it propagates upward through ancestors to the document, so a parent listener can respond to child events.

preventDefault versus stopPropagation

preventDefault cancels the browser's default action, such as following a link; stopPropagation halts the event's travel through the capture or bubble phases.

DOMContentLoaded event

It fires on document after HTML has been parsed and deferred scripts have run, before images and stylesheets finish, so DOM queries are safe there.