René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"'for..in' for null/undefined expression should not be an error in --strictNullChecks","articleBody":"```ts\r\n// @strictNullChecks: true\r\nfunction f(x: { [key: string]: number; } | null | undefined) {\r\n  for (const key in x) {  // 1\r\n    console.log(x[key]);  // 2\r\n  }\r\n}\r\n```\r\n\r\n**Expected behavior:**\r\n\r\nThe `for..in` statement in ECMAScript does not throw if the expression is `undefined` or `null`, but rather the loop body is not evaluated. As such, the correct behavior under `--strictNullChecks` should be:\r\n\r\n- The `for..in` statement (1) should be legal, even if `x` is `undefined` or `null`.\r\n- Inside the `for..in` statement, the `x` (2) should no longer be considered `undefined` or `null`.\r\n\r\n**Actual behavior:**\r\n\r\n- The `for..in` statement (1) reports the error: `Object is possibly 'null' or 'undefined'.`\r\n- Inside the `for..in` statement, the `x` (2) reports the error: `Object is possibly 'null' or 'undefined'.`","author":{"url":"https://github.com/rbuckton","@type":"Person","name":"rbuckton"},"datePublished":"2017-10-25T00:03:26.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/19461/TypeScript/issues/19461"}