[New] add t.intercept()

Author: ljharb

lib/test.js

@@ -245,6 +245,103 @@ Test.prototype.captureFn = function captureFn(original) {
 	return wrapObject.wrapped;
 };
 
+Test.prototype.intercept = function intercept(obj, property) {
+	if (!obj || (typeof obj !== 'object' && typeof obj !== 'function')) {
+		throw new TypeError('`obj` must be an object');
+	}
+	if (typeof property !== 'string' && typeof property !== 'symbol') {
+		throw new TypeError('`property` must be a string or a symbol');
+	}
+	var desc = arguments.length > 2 ? arguments[2] : { __proto__: null };
+	if (typeof desc !== 'undefined' && (!desc || typeof desc !== 'object')) {
+		throw new TypeError('`desc`, if provided, must be an object');
+	}
+	if ('configurable' in desc && !desc.configurable) {
+		throw new TypeError('`desc.configurable`, if provided, must be `true`, so that the interception can be restored later');
+	}
+	var isData = 'writable' in desc || 'value' in desc;
+	var isAccessor = 'get' in desc || 'set' in desc;
+	if (isData && isAccessor) {
+		throw new TypeError('`value` and `writable` can not be mixed with `get` and `set`');
+	}
+	var strictMode = arguments.length > 3 ? arguments[3] : true;
+	if (typeof strictMode !== 'boolean') {
+		throw new TypeError('`strictMode`, if provided, must be a boolean');
+	}
+
+	var calls = [];
+	var getter = desc.get && callBind.apply(desc.get);
+	var setter = desc.set && callBind.apply(desc.set);
+	var value = !isAccessor ? desc.value : void undefined;
+	var writable = !!desc.writable;
+
+	function getInterceptor() {
+		var args = $slice(arguments);
+		if (isAccessor) {
+			if (getter) {
+				var completed = false;
+				try {
+					var returned = getter(this, arguments);
+					completed = true;
+					$push(calls, { type: 'get', success: true, value: returned, args: args, receiver: this });
+					return returned;
+				} finally {
+					if (!completed) {
+						$push(calls, { type: 'get', success: false, threw: true, args: args, receiver: this });
+					}
+				}
+			}
+		}
+		$push(calls, { type: 'get', success: true, value: value, args: args, receiver: this });
+		return value;
+	}
+
+	function setInterceptor(v) {
+		var args = $slice(arguments);
+		if (isAccessor && setter) {
+			var completed = false;
+			try {
+				var returned = setter(this, arguments);
+				completed = true;
+				$push(calls, { type: 'set', success: true, value: v, args: args, receiver: this });
+				return returned;
+			} finally {
+				if (!completed) {
+					$push(calls, { type: 'set', success: false, threw: true, args: args, receiver: this });
+				}
+			}
+		}
+		var canSet = isAccessor || writable;
+		if (canSet) {
+			value = v;
+		}
+		$push(calls, { type: 'set', success: !!canSet, value: value, args: args, receiver: this });
+
+		if (!canSet && strictMode) {
+			throw new TypeError('Cannot assign to read only property \'' + property + '\' of object \'' + inspect(obj) + '\'');
+		}
+		return value;
+	}
+
+	var restore = mockProperty(obj, property, {
+		nonEnumerable: !!desc.enumerable,
+		get: getInterceptor,
+		set: setInterceptor
+	});
+	this.teardown(restore);
+
+	function results() {
+		try {
+			return calls;
+		} finally {
+			calls = [];
+		}
+	}
+	results.restore = restore;
+
+	return results;
+};
+
 Test.prototype._end = function (err) {
 	var self = this;
 	if (this._progeny.length) {

readme.markdown

@@ -396,6 +396,23 @@ The returned wrapper has a `.calls` property, which is an array that will be pop
 
 Modeled after [tap](https://tapjs.github.io/tapjs/modules/_tapjs_intercept.html).
 
+## t.intercept(obj, property, desc = {}, strictMode = true)
+
+Similar to `t.capture()``, but can be used to track get/set operations for any arbitrary property.
+Calling the returned `results()` function will return an array of call result objects.
+The array of calls will be reset whenever the function is called.
+Call result objects will match one of these forms:
+  - `{ type: 'get', value: '1.2.3', success: true, args: [x, y, z], receiver: o }`
+  - `{ type: 'set', value: '2.4.6', success: false, args: [x, y, z], receiver: o }`
+
+If `strictMode` is `true`, and `writable` is `false`, and no `get` or `set` is provided, an exception will be thrown when `obj[property]` is assigned to.
+If `strictMode` is `false` in this scenario, nothing will be set, but the attempt will still be logged.
+
+Providing both `desc.get` and `desc.set` are optional and can still be useful for logging get/set attempts.
+
+`desc` must be a valid property descriptor, meaning that `get`/`set` are mutually exclusive with `writable`/`value`.
+Additionally, explicitly setting `configurable` to `false` is not permitted, so that the property can be restored.
+
 ## var htest = test.createHarness()
 
 Create a new test harness instance, which is a function like `test()`, but with a new pending stack and test state.