Javascript call() function - extra parameters -

Javascript call() function - extra parameters -

i'm reading javascript: definitive guide 6th edition. teaches ecmascript 5. anyway, doesn't explain things thoroughly, call() function example. extent of book's definition:

any arguments call() after first invocation context argument values passed function invoked. example, pass 2 numbers function f() , invoke if method of object o, utilize code this:

f.call(o, 1, 2);

in next section author builds map function. i've been studying ruby know how map works. question implementation using call() function. looks this:

var map = function(a,f, o) { var results = []; for(var = 0, len = a.length; < len; i++) { if (i in a) results[i] = f.call(o || null, a[i], i, a); } homecoming results; };

it defines square function , puts map use:

function square(x){ homecoming x*x; } var array = [1,2,3,4,5]; var results = map(array, square);

what purpose of i, , a parameters in call() function? if remove them same results.

function.call allows phone call function though method attached object.

what means can have function defined somewhere unrelated object, , can phone call function though part of object. long way of saying when utilize function.call, telling js engine utilize first parameter whenever utilize 'this' within function.

so:

function set_field_value(name, value) { // stuff this[name] = value; }

makes no sense itself, because special variable 'this' not set (meaningful) but, if utilize call, can set whatever want:

// if my_object = object: set_field_value.call(my_object, 'firstname', 'bob'); console.log(my_object.firstname); // prints 'bob'

the of import argument phone call first one, (in above case, my_object) because first argument becomes 'this' within function. rest of arguments passed 'as is' function.

so - in example, , arguments there create map function other map functions, provide array (a) , index (i) beingness worked on.

hope helps,

jay

ps - recommend book 'javascript: parts' - makes lot more sense definitive guide.

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