Ternary operator example
let status = age >= 18 ? "adult" : "minor";
What is variable hoisting
- Variables (
var) in JavaScript are, in a sense, “hoisted” (or “lifted”) to the top of the function or statement. - However, variables that are hoisted return a value of undefined (cuz why not xd)
- “Temporal Dead Zone” (TDZ) - when variable is observable but unusable variables.
- Because of hoisting, all var statements in a function should be placed as near to the top of the function as possible.
letandconstare hoisted but not initialized. Referencing the variable in the block before the variable declaration results in aReferenceError.
Convert variable to Boolean
Two ways
Boolean(-1);!!value
Convert variable to Number
Number(" -12.34 ");
Convert variable to String
String(12.34);${x}toString() if supported on object
Number literal for hex, binary, BigNum numbers
0xff, 0b1101, 123n
Convert string to Float
parseFloat("12.5");
Convert string to Number
parseInt("101", 2); // binary to number
Conditionals: if
if (condition_1) {
statement_1;
} else if (condition_2) {
statement_2;
} else {
statement_last;
}Conditionals: switch
switch (expression) {
case label_1:
statements_1;
break;
default:
statements_def;
break;
}Conditionals: for loop (C like)
for (let i = 0; i < selectObject.options.length; i++) {
// index for - use by default
}
for (var i = 0, j = 9; i <= j; i++, j--) {
// Two variables at once
}Conditionals: While loop(s)
while (n < 3) {
n++;
x += n;
}
do {
i += 1;
console.log(i);
} while (i < 5);Conditionals: For each loop(s)
Creates a loop Iterating over iterable objects (including Array, Map, Set, arguments object and so on), invoking a custom iteration hook:
for (let i of arr) {
console.log(i);
}Will iterate all properties (numeric indexes + custom properties) or methods you define on Array object (or other object):
for (let i in obj) {
console.log(i);
}Exception handling structure
try {
throw new Error("The message");
} catch (e) {
throw e; // re-throw
} finally {
console.log("Resource cleanup...");
}JS Exceptions:
* How to throw
* Exception object properties
* How to create custom exception
- How to throw:
throw new Error("The message");
- Exception object properties:
console.error(e.name);console.error(e.message);
- How to create custom exception:
class CustomError extends Error { constructor(message) { super(message); // Pass message to the Error class this.name = "CustomError"; // Set a custom error name } }
What happens when you return value in finally block?
It overwrites previous return (including one in catch block)
What is a closure. Use cases.
Closure is when a function remembers and continues to access variables from outside its scope, even when the function is executed in a different scope.
A closure is the combination of a function and the lexical environment within which that function was declared.
The Lexical Environment is a structure that holds identifier-variable mapping
Use cases:
* Data Encapsulation (Private Variables)
* Function Factories
* Memoization (Performance Optimization)
* Event Handlers (Retaining State)
* Iterators
* Partial Application / Currying
Variadic function parameters
function multiply(multiplier, ...theArgs) {
// theArgs is an actual array
return theArgs.map((x) => multiplier * x);
}Renamed export
export {function1 as newFunctionName};
Export from another module
export { name } from 'x.js'
Renamed import
import {function1 as newFunctionName} from "x"
- Parse JSON string to object.
- Convert object to JSON string
- Convert object to JSON string, use 2 spaces for indentation (what are all parameters)
JSON.parse(jsonString)JSON.stringify(obj)JSON.stringify(value, undefined, 2)
Second arg is replacer - a function that alters the behavior of the stringification process
How to handle cases where dog in adventurer.dog.name can be an undefined object? Name the techniques.
- Use
??(nullish coalescing - scalania) and?.(optional chaining)adventurer.dog?.name ?? "backup value"
a.b?.cwill return undefined insatead of Error?? "backup value"will return"backup value"only if the calculated value is undefined.
Check typeof variable x
typeof x
