Javascript Assertion
by Geethalakshmi[ Edit ] 2010-09-16 21:23:36
Javascript Assertion
Assertion is one of the commonly-used debugging techniques. It's used to ensure that an expression evaluates to true during execution. if the expression evaluates to false, this indicates a possible bug in code. JavaScript lacks a built-in assert function, but fortunately it's easy to write one. The following implementation throws an exception of type AssertException if the passed expression evaluates to false:
function AssertException(message) { this.message = message; }
AssertException.prototype.toString = function () {
return 'AssertException: ' + this.message;
}
function assert(exp, message) {
if (!exp) {
throw new AssertException(message);
}
}
Throwing an exception on its own isn't very useful, but when combined with a helpful error message or a debugging tool, you can detect the problematic assertion. You may also check whether an exception is an assertion exception by using the following snippet:
try {
// ...
}
catch (e) {
if (e instanceof AssertException) {
// ...
}
}
This function can be used in a way similar to C or Java:
assert(obj != null, 'Object is null');
If obj happens to be null, the following message will be printed in the JavaScript console in Firefox:
uncaught exception: AssertException: Object is null
