Add support for toJSON

This brings `jsesc(data, { 'json': true })` closer to `JSON.stringify(data)`.

Fixes #17.

jsesc.js

@@ -51,13 +51,20 @@
 		return toString.call(value) == '[object Array]';
 	};
 	var isObject = function(value) {
-		// simple, but good enough for what we need
+		// This is a very simple check, but it’s good enough for what we need.
 		return toString.call(value) == '[object Object]';
 	};
 	var isString = function(value) {
 		return typeof value == 'string' ||
 			toString.call(value) == '[object String]';
 	};
+	var isFunction = function(value) {
+		// In a perfect world, the `typeof` check would be sufficient. However,
+		// in Chrome 1–12, `typeof /x/ == 'object'`, and in IE 6–8
+		// `typeof alert == 'object'` and similar for other host objects.
+		return typeof value == 'function' ||
+			toString.call(value) == '[object Function]';
+	};
 
 	/*--------------------------------------------------------------------------*/
 
@@ -71,7 +78,7 @@
 		'\n': '\\n',
 		'\r': '\\r',
 		'\t': '\\t'
-		// `\v` is omitted intentionally, because in IE < 9, '\v' == 'v'
+		// `\v` is omitted intentionally, because in IE < 9, '\v' == 'v'.
 		// '\v': '\\x0B'
 	};
 	var regexSingleEscape = /["'\\\b\f\n\r\t]/;
@@ -108,6 +115,10 @@
 		var result;
 		var isEmpty = true;
 
+		if (json && argument && isFunction(argument.toJSON)) {
+			argument = argument.toJSON();
+		}
+
 		if (!isString(argument)) {
 			if (isArray(argument)) {
 				result = [];

src/jsesc.js

@@ -51,13 +51,20 @@
 		return toString.call(value) == '[object Array]';
 	};
 	var isObject = function(value) {
-		// simple, but good enough for what we need
+		// This is a very simple check, but it’s good enough for what we need.
 		return toString.call(value) == '[object Object]';
 	};
 	var isString = function(value) {
 		return typeof value == 'string' ||
 			toString.call(value) == '[object String]';
 	};
+	var isFunction = function(value) {
+		// In a perfect world, the `typeof` check would be sufficient. However,
+		// in Chrome 1–12, `typeof /x/ == 'object'`, and in IE 6–8
+		// `typeof alert == 'object'` and similar for other host objects.
+		return typeof value == 'function' ||
+			toString.call(value) == '[object Function]';
+	};
 
 	/*--------------------------------------------------------------------------*/
 
@@ -71,7 +78,7 @@
 		'\n': '\\n',
 		'\r': '\\r',
 		'\t': '\\t'
-		// `\v` is omitted intentionally, because in IE < 9, '\v' == 'v'
+		// `\v` is omitted intentionally, because in IE < 9, '\v' == 'v'.
 		// '\v': '\\x0B'
 	};
 	var regexSingleEscape = /["'\\\b\f\n\r\t]/;
@@ -108,6 +115,10 @@
 		var result;
 		var isEmpty = true;
 
+		if (json && argument && isFunction(argument.toJSON)) {
+			argument = argument.toJSON();
+		}
+
 		if (!isString(argument)) {
 			if (isArray(argument)) {
 				result = [];

tests/tests.js

@@ -263,6 +263,26 @@
 			'"foo\\u{1D306}bar\\u00A9baz"',
 			'Escaping as JSON with `es6: true`'
 		);
+		var tmp = {
+			'shouldn\u2019t be here': 10,
+			'toJSON': function() {
+				return {
+					'hello': 'world',
+					'\uD83D\uDCA9': 'foo',
+					'pile': '\uD83D\uDCA9'
+				};
+			}
+		};
+		equal(
+			jsesc(tmp, { 'json' : true }),
+			'{"hello":"world","\\uD83D\\uDCA9":"foo","pile":"\\uD83D\\uDCA9"}',
+			'`toJSON` methods are called when `json: true`'
+		);
+		notEqual(
+			jsesc(tmp),
+			'{"hello":"world","\\uD83D\\uDCA9":"foo","pile":"\\uD83D\\uDCA9"}',
+			'`toJSON` methods are not called when `json: false`'
+		);
 	});
 
 	if (runExtendedTests) {