ECMAScript 2018 Standard Method
You would use object spread:
let merged = {...obj1, ...obj2};
merged
is now the union of obj1
and obj2
. Properties in obj2
will overwrite those in obj1
.
/** There's no limit to the number of objects you can merge.
* Later properties overwrite earlier properties with the same name. */
const allRules = {...obj1, ...obj2, ...obj3};
Here is also the MDN documentation for this syntax. If you're using babel you'll need the babel-plugin-transform-object-rest-spread plugin for it to work.
ECMAScript 2015 (ES6) Standard Method
/* For the case in question, you would do: */
Object.assign(obj1, obj2);
/** There's no limit to the number of objects you can merge.
* All objects get merged into the first object.
* Only the object in the first argument is mutated and returned.
* Later properties overwrite earlier properties with the same name. */
const allRules = Object.assign({}, obj1, obj2, obj3, etc);
(see MDN JavaScript Reference)
Method for ES5 and Earlier
for (var attrname in obj2) { obj1[attrname] = obj2[attrname]; }
Note that this will simply add all attributes of obj2
to obj1
which might not be what you want if you still want to use the unmodified obj1
.
If you're using a framework that craps all over your prototypes then you have to get fancier with checks like hasOwnProperty
, but that code will work for 99% of cases.
Example function:
/**
* Overwrites obj1's values with obj2's and adds obj2's if non existent in obj1
* @param obj1
* @param obj2
* @returns obj3 a new object based on obj1 and obj2
*/
function merge_options(obj1,obj2){
var obj3 = {};
for (var attrname in obj1) { obj3[attrname] = obj1[attrname]; }
for (var attrname in obj2) { obj3[attrname] = obj2[attrname]; }
return obj3;
}
The difference is that functionOne
is a function expression and so only defined when that line is reached, whereas functionTwo
is a function declaration and is defined as soon as its surrounding function or script is executed (due to hoisting).
For example, a function expression:
// TypeError: functionOne is not a function
functionOne();
var functionOne = function() {
console.log("Hello!");
};
And, a function declaration:
// Outputs: "Hello!"
functionTwo();
function functionTwo() {
console.log("Hello!");
}
Historically, function declarations defined within blocks were handled inconsistently between browsers. Strict mode (introduced in ES5) resolved this by scoping function declarations to their enclosing block.
'use strict';
{ // note this block!
function functionThree() {
console.log("Hello!");
}
}
functionThree(); // ReferenceError
Best Answer
slice()
works likesubstring()
with a few different behaviors.What they have in common:
start
equalsstop
: returns an empty stringstop
is omitted: extracts characters to the end of the stringDistinctions of
substring()
:start > stop
, thensubstring
will swap those 2 arguments.NaN
, it is treated as if it were0
.Distinctions of
slice()
:start > stop
,slice()
will return the empty string. (""
)start
is negative: sets char from the end of string, exactly likesubstr()
in Firefox. This behavior is observed in both Firefox and IE.stop
is negative: sets stop to:string.length – Math.abs(stop)
(original value), except bounded at 0 (thus,Math.max(0, string.length + stop)
) as covered in the ECMA specification.Source: Rudimentary Art of Programming & Development: Javascript: substr() v.s. substring()