General info
We use ES6 (EcmaScript standard version 6) |
Runs natively on browsers AND on a computer (using Node.js). Execution environments: Server & CLI Node.js, Browser, learning aids ( Python Tutor). |
JS engines (interpreters): V8, SpiderMonkey, JavaScriptCore. |
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First line of file is "use strict" ;
Can't define properties/parameters with same name; |
Control Structures
If conditions: if - else if - else
, switch (expr)
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Loops: for (initial_expr; cond; increment;)
, do - while
, while
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Special for statements: for (var in object) {}
: iterates over all the enumerable properties of an object. Do not use to traverse an array. for (var of iterable) {}
: iterates the variable over all values of an iterable object (array, map, etc.) and returns the values. |
Exception handling: try-catch-finally
. Ready to use throwables. |
The condition of the ifs causes an implicit conversion of whatever is written to a boolean.
The expression in the switch may also be a string.
In loops, we may use break;
or continue;
Expressions
Declare + initialize: let variable = expression ;
Reassign: variable = expression ;
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Comparison: convert and compare: a == b
same type and value: a === b
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Conversions: any to boolean: truthy-falsy rule, Boolean(a), !!a
String to Number: Number(s), parseInt(s), parseFloat(s)
Number to String: n.toString(), String(n), n+""
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String concatenation: string1 + string2
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Default value assignment ( if a then a else b): a || b
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Strings
Immutable sequence of unicode characters. All operations always return new strings. |
Length = # of characters (not bytes).
Empty string has length 0 and is a falsy. |
Operations: indexing s[3]
concatenation s1 + s2
# of characters s.length
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Template literals ("dynamic string concatenation") let name = "Bill";
let greeting = `Hello ${ name }.`;
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Some Unicode characters are represented by two code units, so some string methods above FFFF might misbehave.
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Language Structure
One file = one JS program
((loaded independently but communicate w/ global state and modules) |
File is entirely parsed and then executed top to bottom. |
Written in Unicode, case sensitive. |
Types and Variables
Values have types:
"type" is a property of a value. {{nl}]
Variables DON'T have types:
variables can contain any type, and ≠ types in ≠ moments. |
Boolean type: 'true' or 'false' literal values Conversion: Truthies: 0, -0, NaN, undefined, null, ' '
Falsies: 3, 'false', [], {}, ...
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Numbers:
No distinction integers and reals
Automatic conversion according to operation |
Nullish values: undefined
: variable declared but not initialized. Returned by void functions. null
: empty value |
Variables: They're pure references: refer to a value Declare: let, const, var. Let
: yes reassign, no redeclare, block scope, no hoisting Const
: no reassign, no redeclare, block scope, no hoisting Var
: yes reassign and redeclare, function/global scope, hoisting. {{ln}} {{ln}} Block scope: variable exists only in defined and inner scopes. {{ln}} Hoisting: declaration of var inside code is moved to top of scope. |
Arrays
Elements do not need to be of the same type. |
Have property length
(automatic). |
Create arrays using parameters: let v = Array.of(1, 2, 3) ;
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Add elements: let v = [] ; v[0] = "a" ; v[1] = 8 ;
.push()
adds to end of array .unshift()
adds to beginning of array. .length
adjusts automatically. Removal: .unshift()
and .pop()
. |
Copy of the reference: let v = []; let alias = v;
we establish alias
as an alias of v
, so if we modify alias
we're actually modifying v
. |
Shallow copy of arrays: let copy = Array.from(v);
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Destructuring assignment : Value of the right are extracted and stored in the variables on the left. [x,y] = [y,x];
easy swap. |
Spread operator : "all the rest": let [x, ...y] = [1,2,3,4];·
we obtain y == [2,3,4] "everything inside vector x". Can be used to copy arrays by value: const b = Array.of(...a)
, const b = [...a]
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Automatic expansion of array: let v = []; v[3] = "a"
.
Arrays are not values, they're references.
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