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Function Object

A function object is evaluated each time it is used, which is less efficient than calling pre-compiled function declarations.

Properties: length returns the number of parameters expected by a function. Within any function body, you can use the arguments arrayto access the parameters.


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<!DOCTYPE html><html><body><script>

function S(){
   var sum=0;
   for (i=0; i<arguments.length; i++){
      sum+=arguments[i];
   }
   console.log(arguments.callee);  // shows the function definition
   return sum;}console.log(S(10,20,30));  // … 60console.log((function(a,b,c){}).length);  // 3

</script></body><html>

Methods: call( o1[,o2......] ) calls the function by passing o1 as the 'this' value, and subsequent arguments as the parameters. apply(o[,arr]) is similar to call() but passes the arguments as an array instead.


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<!DOCTYPE html><html><body><script>

var animals = [ { species: 'Lion', name: 'King' },
                         { species: 'Whale', name: 'Willy' }];for (let i = 0; i < animals.length; i++) {
   (function(i) {
      console.log('#' + i + ' ' + this.species + ': ' +
                        this.name);
   }).call(animals[i], i);}// #0 Lion: King// #1 Whale: Willy

</script></body><html>

bind(o1[,o2......]) returns a new function by passing o1 as the 'this' value, and subsequent arguments as the parameters.


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<!DOCTYPE html><html><body><script>

function sum(){
   var sum=0;
   for (i=0; i<arguments.length; i++){
      sum+=arguments[i];
   }
   return sum;}console.log(sum(10,20,30));  // 60plus1000 = sum.bind(null,1000);console.log(plus1000(10,20,30));  // 1060

</script></body><html>

toString() returns a string representation of the function.


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<!DOCTYPE html><html><body><script>

function sum(a,b){return a+b;}console.log(sum.toString());// function sum(a,b){return a+b;}

</script></body><html>

Below are two ways to execute a string of JavaScript code:


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<!DOCTYPE html><html><body><script>

var code = "console.log(100);"var f = Function(code); f(); // 100eval(code); // 100

</script></body><html>