Javascript Blind Spot
This is a collection of javascript blind spot.
Array
- map parameters
['1', '2', '3'].map(parseInt) // [1, NaN, NaN]
['1', '2', '3'].map((currentValue, index, array) => {
console.log(currentValue)
console.log(index)
console.log(array)
})
// parseInt(1, 0) == parseInt(1, 10) 1
// parseInt(2, 1) NaN
// parseInt(3, 2) NaN
- map will only called by initialized array element
var ary = Array(3);
ary[0]=2
ary.map(function(elem) { return '1'; });
// ["1", empty × 2]
- filter will not work on empty elements
var ary = [0,1,2];
ary[10] = 10;
ary.filter(function(x) { return x === undefined;});
- array < > will compare element ony by one but == and === will not
var a = [1, 2, 3],
b = [1, 2, 3],
c = [1, 2, 4]
a == b // false
a === b // false
a > c // false
a < c // true
- array allow last one be comma
Operation precedence
| Precedence | Operator type | Associativity | Individual operators | |
|---|---|---|---|---|
| 20 | Grouping | n/a | ( … ) | |
| 19 | Member Access | left-to-right | … . … | |
| Computed Member Access | left-to-right | … [ … ] | ||
| new (with argument list) | n/a | new … ( … ) | ||
| Function Call | left-to-right | … ( … ) | ||
| 18 | new (without argument list) | right-to-left | new … | |
| 17 | Postfix Increment | n/a | … ++ | |
| Postfix Decrement | … – | |||
| 16 | Logical NOT | right-to-left | ! … | |
| Bitwise NOT | ~ … | |||
| Unary Plus | + … | |||
| Unary Negation | - … | |||
| Prefix Increment | ++ … | |||
| Prefix Decrement | – … | |||
| typeof | typeof … | |||
| void | void … | |||
| delete | delete … | |||
| await | await … | |||
| 15 | Exponentiation | right-to-left | … ** … | |
| 14 | Multiplication | left-to-right | … * … | |
| Division | … / … | |||
| Remainder | … % … | |||
| 13 | Addition | left-to-right | … + … | |
| Subtraction | … - … | |||
| 12 | Bitwise Left Shift | left-to-right | … « … | |
| Bitwise Right Shift | … » … | |||
| Bitwise Unsigned Right Shift | … »> … | |||
| 11 | Less Than | left-to-right | … < … | |
| Less Than Or Equal | … <= … | |||
| Greater Than | … > … | |||
| Greater Than Or Equal | … >= … | |||
| in | … in … | |||
| instanceof | … instanceof … | |||
| 10 | Equality | left-to-right | … == … | |
| Inequality | … != … | |||
| Strict Equality | … === … | |||
| Strict Inequality | … !== … | |||
| 9 | Bitwise AND | left-to-right | … & … | |
| 8 | Bitwise XOR | left-to-right | … ^ … | |
| 7 | Bitwise OR | left-to-right | … | … | |
| 6 | Logical AND | left-to-right | … && … | |
| 5 | Logical OR | left-to-right | … || … | |
| 4 | Conditional | right-to-left | … ? … : … | |
| 3 | Assignment | right-to-left | … = … | |
| … += … | ||||
| … -= … | ||||
| … **= … | ||||
| … *= … | ||||
| … /= … | ||||
| %= … | ||||
| … «= … | ||||
| … »= … | ||||
| … »>= … | ||||
| … &= … | ||||
| … ^= … | ||||
| … |= … | ||||
| 2 | yield | right-to-left | yield … | |
| yield* | yield* … | |||
| 1 | Comma \ Sequence | left-to-right | … , … |
var val = 'smtg';
console.log('Value is ' + (val === 'smtg') ? 'Something' : 'Nothing'); // Something
String
- new String will return String object
var a = new String('a')
var b = String('a')
a === 'a' // false
b === 'a' // true
== vs ===
- can not compare regex – every regex is unique
var a = /123/, b = /123/;
a == b // false
a === b // false
- == will perform type convent
false == 0 // true
false == '0' // true
undefined == null // true
prototype vs proto vs Object.getPrototypeOf
-
prototype is a property of a Function object. It is the prototype of objects constructed by that function.
-
proto is internal property of an object, pointing to its prototype. Current standards provide an equivalent Object.getPrototypeOf(O) method, though de facto standard proto is quicker
-
Instanceof relationships by comparing a function’s prototype to an object’s proto chain, and you can break these relationships by changing prototype.
function Test() {
}
var t1 = new Test()
// below all true
t1.__proto__ === Test.prototype
t1.__proto__.__proto__ === Object.prototype
t1 instanceof Test
t1 instanceof Object
typeof vs instanceof
typeof null // "object"
null instanceof Object // false
Object.prototype.toString.call(null) // '[Object Null]'
typeof [] // "object"
[] instanceof Object // true
Object.prototype.toString.call([]) // '[Object, Array]'
hasOwnProperty will never look for proto chain
function
- function name is read only