Initial implementation

.eslintrc

@@ -2,4 +2,10 @@
 	"root": true,
 
 	"extends": "@ljharb",
+
+	"rules": {
+		"max-lines-per-function": 0,
+		"max-params": 0,
+		"new-cap": [2, { "capIsNewExceptions": ["GetIntrinsic"] }],
+	},
 }

index.js

@@ -0,0 +1,107 @@
+'use strict';
+
+var GetIntrinsic = require('es-abstract/GetIntrinsic');
+var callBound = require('es-abstract/helpers/callBound');
+var inspect = require('object-inspect');
+
+var $TypeError = GetIntrinsic('%TypeError%');
+var $WeakMap = GetIntrinsic('%WeakMap%', true);
+var $Map = GetIntrinsic('%Map%', true);
+var $push = callBound('Array.prototype.push');
+
+var $weakMapGet = callBound('WeakMap.prototype.get', true);
+var $weakMapSet = callBound('WeakMap.prototype.set', true);
+var $weakMapHas = callBound('WeakMap.prototype.has', true);
+var $mapGet = callBound('Map.prototype.get', true);
+var $mapSet = callBound('Map.prototype.set', true);
+var $mapHas = callBound('Map.prototype.has', true);
+var objectGet = function (objects, key) { // eslint-disable-line consistent-return
+	for (var i = 0; i < objects.length; i += 1) {
+		if (objects[i].key === key) {
+			return objects[i].value;
+		}
+	}
+};
+var objectSet = function (objects, key, value) {
+	for (var i = 0; i < objects.length; i += 1) {
+		if (objects[i].key === key) {
+			objects[i].value = value; // eslint-disable-line no-param-reassign
+			return;
+		}
+	}
+	$push(objects, {
+		key: key,
+		value: value
+	});
+};
+var objectHas = function (objects, key) {
+	for (var i = 0; i < objects.length; i += 1) {
+		if (objects[i].key === key) {
+			return true;
+		}
+	}
+	return false;
+};
+
+module.exports = function getSideChannel() {
+	var $wm;
+	var $m;
+	var $o;
+	var channel = {
+		assert: function (key) {
+			if (!channel.has(key)) {
+				throw new $TypeError('Side channel does not contain ' + inspect(key));
+			}
+		},
+		get: function (key) { // eslint-disable-line consistent-return
+			if ($WeakMap && key && (typeof key === 'object' || typeof key === 'function')) {
+				if ($wm) {
+					return $weakMapGet($wm, key);
+				}
+			} else if ($Map) {
+				if ($m) {
+					return $mapGet($m, key);
+				}
+			} else {
+				if ($o) { // eslint-disable-line no-lonely-if
+					return objectGet($o, key);
+				}
+			}
+		},
+		has: function (key) {
+			if ($WeakMap && key && (typeof key === 'object' || typeof key === 'function')) {
+				if ($wm) {
+					return $weakMapHas($wm, key);
+				}
+			} else if ($Map) {
+				if ($m) {
+					return $mapHas($m, key);
+				}
+			} else {
+				if ($o) { // eslint-disable-line no-lonely-if
+					return objectHas($o, key);
+				}
+			}
+			return false;
+		},
+		set: function (key, value) {
+			if ($WeakMap && key && (typeof key === 'object' || typeof key === 'function')) {
+				if (!$wm) {
+					$wm = new $WeakMap();
+				}
+				$weakMapSet($wm, key, value);
+			} else if ($Map) {
+				if (!$m) {
+					$m = new $Map();
+				}
+				$mapSet($m, key, value);
+			} else {
+				if (!$o) {
+					$o = [];
+				}
+				objectSet($o, key, value);
+			}
+		}
+	};
+	return channel;
+};

package.json

@@ -34,5 +34,9 @@
 		"@ljharb/eslint-config": "^15.0.2",
 		"eslint": "^6.7.2",
 		"tape": "^4.11.0"
+	},
+	"dependencies": {
+		"es-abstract": "^1.16.2",
+		"object-inspect": "^1.7.0"
 	}
 }