its

Its a utility to simplify common precondition or state checking. It's useful for signaling to calling methods when they've made invalid calls to a method.

Usage

There are four available functions:

• its(expression [, errorType] [, messageTemplate [, messageArgs...]]) for throwing custom errors
• its.defined(expression [, messageTemplate [, messageArgs...]]) for throwing reference errors
• its.range(expression [, messageTemplate [, messageArgs...]]) for throwing range errors
• its.type(expression [, messageTemplate [, messageArgs...]]) for throwing type errors
  • its.undefined(obj [, messageTemplate [, messageArgs...]]) for throwing type errors
  • its.null(obj [, messageTemplate [, messageArgs...]]) for throwing type errors
  • its.boolean(obj [, messageTemplate [, messageArgs...]]) for throwing type errors
  • its.array(obj [, messageTemplate [, messageArgs...]]) for throwing type errors
  • its.object(obj [, messageTemplate [, messageArgs...]]) for throwing type errors
  • its.function(obj [, messageTemplate [, messageArgs...]]) for throwing type errors
  • its.string(obj [, messageTemplate [, messageArgs...]]) for throwing type errors
  • its.number(obj [, messageTemplate [, messageArgs...]]) for throwing type errors
  • its.date(obj [, messageTemplate [, messageArgs...]]) for throwing type errors
  • its.regexp(obj [, messageTemplate [, messageArgs...]]) for throwing type errors

expression is a boolean value which determines whether the precondition will throw an error or not.

messageTemplate is a message with 0 or more '%s' placeholders for message arguments

messageArgs is a variable argument (0 or more) to fill the placeholders in the message template

errorType is used for throwing custom error objects. These objects should inherit from Error.

Examples

// Things that should pass
its.string('hi'); // returns true
its.function(function(){}); // returns true
its.date(new Date); //returns true
its.defined("anything"); // returns "anything"
its.type(typeof "something" === "string"); // returns true
its.range(1 < 2 && 1 > 0); // returns true
its(1 === 1); // returns true
its(1 === 1, ReferenceError); // throws true

// Things that shouldn't pass
its.defined(void 0); // throws ReferenceError
its.type(typeof "something" === "number"); // throws TypeError
its.range(1 < 2 && 1 > 2); // throws RangeError
its(1 !== 1); // throws Error
its(1 === void 0, ReferenceError); // throws ReferenceError

// Messages
its.defined(void 0, "This doesn't look right."); // throws ReferenceError with a message of "This doesn't look right."
its.defined(void 0, "This doesn't look %s.", "right"); // throws ReferenceError with a message of "This doesn't look right."
its.defined(void 0, "%s doesn't look %s.", "This", "right"); // throws ReferenceError with a message of "This doesn't look right."

// What real use may look like
var addOnlyNumbersBelow100 = function(number1, number2){
	its.number(number1);
	its.number(number2);
	its.range(number1 < 100);
	its.range(number2 < 100);
	return number1 + number2;
};

addOnlyNumbersBelow100(10, 20); // returns 30
addOnlyNumbersBelow100(10, 338484); // throws RangeError
addOnlyNumbersBelow100("10", 338484); // throws TypeError

Developing

its uses grunt to build.

• grunt - Builds the standard and minified version of precondition in the dist folder
• grunt test - Builds precondition and runs unit tests (requires PhantomJS)