Sundarrajk's Weblog

JavaScript – The Good Parts – Part 6

Posted on: January 14, 2013


This is the penultimate posting in this series of posts based on the book JavaScript – The Good Parts by Douglas Crockford.
The previous post covered how to “curry” functions and some JavaScript gotchas.
This post gives a list of pre-built methods in the standard objects pre-defined in JavaScript. Many of these would be known only to the hardcore users of JavaScripts and this is serve as an eye-opener to the casual users of JavaScript.

Array
array.concat(items)
This method appends the items to the base array. If the items itself is an array that elements of that array are copied to that of the base array.
E.g.
var a = [‘a’, ‘b’, ‘c’];
var b = [‘d’, ‘e’, ‘f’];
var c = a.concat(b, true);
//c is [‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, true];

array.join(separator)
This joins the elements in an array separated by the separator and gives a String.
var a = [‘a’, ‘b’, ‘c’];
var c = a.join(“/”);  //c is ‘a/b/c/’

array.pop
This removes the last element from the array and returns this as the value. The size of array reduces by one.
var a = [‘a’, ‘b’, ‘c’];
var c = a.pop();
//c = ‘c’ and a =[‘a’, ‘b’];

array.push(item …)
This adds the object to the end of the array.
var a = [‘a’, ‘b’, ‘c’];
var b = [‘d’, ‘e’, ‘f’];
var c = a.push(b, true);
//c is [‘a’, ‘b’, ‘c’, [‘d’, ‘e’, ‘f’], true];
Note how this is different from concat. The array “b” has been embedded at index 3 of the array “a”. It has not copied the contents of array “b” to “a”.

array.reverse()
This reverses the values in the array.
var a = [‘a’, ‘b’, ‘c’];
var b = a.reverse();
//both a and b will be [‘c’, ‘b’, ‘a’]

array.shift()
The shift method removes the first element from an array and returns it. If the array is empty it returns undefined.
var a = [‘a’, ‘b’, ‘c’];
var b = a.shift();
//b is ‘a’ and a is [‘b’,’c’]

array.slice(start, end)
This is like substring. Where start is the start position and end is the last but one position that should be copied. If none is specified then it is copied to the end of the array.
var a = [‘a’, ‘b’, ‘c’];
var b = a.slice(0,1); //b will be [‘a’];
var c= a.slice(1); //c will be [‘b’, ‘c’]
var d = a.slice(1, 2); //d will be [‘b’]

array.sort(comparefn)
This sorts the array assuming that all the elements are string. If one needs to sort based on some other datatype one can pass a compare function. This function will be passed the two consecutive values and it should return a “-1” if the first is less than the second a “0” if they are equal and a “1” if the first is greater than the second for sorting the elements in an ascending fashion.

array.splice(start, deleteCount, item, …)
This method starts deleting “deleteCount” elements from the “start” position and inserts the values in item into this gap.

array.unshift(item …)
The unshift method is like the push method, except that it pushes the items onto the front of the array instead of at the end. It returns the array’s new length.
var a = [‘a’, ‘b’, ‘c’];
var r = a.unshift(‘?’, ‘@’);
//a is [‘?’, ‘@’ ,‘a’, ‘b’, ‘c’];
//r is 5

Function
function.apply(thisArg, arguments)
This executes the function with “thisArg” treated as the “this” for the function and the “arguments” is passed as the “arguments” to the function.

Number
number.toExponential(fractionDigits)
This converts this number to a string in the exponential form. The optional fractionalDigits controls the number of decimal places. This should be between 0 and 20.
Math.PI.toExponential(0) = 3e+0
Math.PI.toExponential(2) = 3.14e+0
Math.PI.toExponential(7) = 3.1415927e+0
Math.PI.toExponential(16) = 3.1415926535897930e+0
Math.PI.toExponential() = 3.141592653589793e+0

number.toFixed(fractionDigits)
This method converts this number to a string in the decimal form. The optional fractionDigits parameter controls the number of decimal places. It should be between 0 and 20. The default is 0.
Math.PI.toFixed(0) = 3
Math.PI.toFixed(2) = 3.14
Math.PI.toFixed(7) = 3.1415927
Math.PI.toFixed(16) = 3.1415926535897930
Math.PI.toFixed() = 3

number.toPrecision(precision) This converts the number to a string in decimal form. The optional precision parameter controls the number of digits of precision. It should be between 1 and 21.
Math.PI.toPrecision(2) = 3.1
Math.PI.toPrecision(7) = 3.141593
Math.PI.toPrecision(16) = 3.141592653589793
Math.PI.toPrecision() = 3.141592653589793

number.toString(radix) This converts the number to string. The optional radix parameter controls the radix or the base. This should be between 2 and 36. The default radix base is 10.
Math.PI.toString(2) = 11.001001000011111101101010100010001000010110100011
Math.PI.toString(7) = 3.1103755242102643
Math.PI.toString(16) = 3.243f6a8885a3
Math.PI.toString() = 3.141592653589793

Object
object.hasOwnProperty(name)
This indicates if the property by the specified “name” belongs the object or is it derived from the one of the prototype objects in the hierarchy.
var a = {member: true};
var b = Object.beget(a); //b uses a as the prototype
var t = a.hasOwnProperty(‘member’); //”t” is true
var f = b.hasOwnProperty(‘member’); //”f” is false
var v = b.member; //“v” is true.

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