JavaScript Learning notes 2-functions

2. function

2.1 Function Direct Quantity

The direct amount of a function is used as an expression, not as a statement, and it does not need to specify a function name. The comparison with the function:

JS Code

function　f(){}
var　f　=　function(){}

2.2 Parameters of the function

JS in the function of the number and type of parameters can be very arbitrary, which needs to be used for the actual situation of the corresponding judgement processing.

Optional parameters

If the parameter of a function is optional, then if it is not transmitted, then it is the value of underfined, in the function Assign a default value to it. This kind of writing is very common.

Use an object as a parameter to a function

When the parameters of the function are longer used, and often with the | | Used together.

Parameter type

If you have strict requirements for the parameter types of a function, be sure to judge the type within the function, and if the type is incorrect, throw an exception to report the fact. such as the Arraycopy () method, the expected argument is an array, and if the incoming is not an array, it needs to be thrown.

Again, for example, we calculate the sum of all elements of an array, and if so, in the previous case, in the array, it is predetermined that all elements are numeric and then summed according to the elements of the value. However, many times the developer is not a person, perhaps the array you are dealing with is provided to you by someone else, or if you are taking data from a database to construct a number of groups, you cannot judge the element as a numeric value, and you must judge whether the incoming is an array and whether the element is numeric. If the element is ignored when underfined or null, or if the element contains a hint when it is not a value, or when the value passed in is not an array. The specific code is as follows:

JS Code

function　ArrayUserCommMgr{}
ArrayUserCommMgr.prototype={
　isArray:function(a){
　　var　result　=　false;
　　if((a　instanceof　Array)||(a　&&　typeof　a　==　"object"　&&　"length"　in　a)){
　　result　=　true;
　}
　return　result;
　},
　getSum:function(a){
　if　(arrayUserCommMgr.isArray()){
　　var　total　=　0;
　　for(var　i　=　0;　i　<　a.length;　i++)　{
　　var　element　=　a[i];
　　if　(!element)　continue;　//　ignore　null　and　undefined　elements
　　if　(typeof　element　==　"number"){
　　　total　+=　element;
　　}else{
　　　throw　new　Error("sum():　all　array　elements　must　be　numbers");
　　}
　　}
　}else{
　　throw　new　Error("sum():　argument　must　be　an　array");
　}
　}
}

The above code notes:

1, with ((a instanceof Array) | |

(a && typeof a = = "Object" && "Length" in a))

To determine whether to pass in an array (instanceof array), or an array-like object ((a && typeof a = = "Object" && "Length" in a)