René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Downlevel emit for template strings should use 'void 0' not 'undefined'","articleBody":"**TypeScript Version:**  3.9.x, 4.0.x-dev\r\n\r\n\u003c!-- Search terms you tried before logging this (so others can find this issue more easily) --\u003e\r\n**Search Terms:** template string emit\r\n\r\n**Code**\r\n\r\nFrom `~/tests/cases/conformance/es2018/invalidTaggedTemplateEscapeSequences.ts`:\r\n```ts\r\nconst x = tag`\\u{hello} ${ 100 } \\xtraordinary ${ 200 } wonderful ${ 300 } \\uworld`;\r\n```\r\n\r\n**Expected behavior:**\r\n\r\nES5/ES3 emit:\r\n```js\r\nconst x = tag(__makeTemplateObject([void 0, void 0, \" wonderful \", void 0], [\"\\\\u{hello} \", \" \\\\xtraordinary \", \" wonderful \", \" \\\\uworld\"]), 100, 200, 300);\r\n```\r\n\r\n**Actual behavior:**\r\n\r\nES5/ES3 emit:\r\n```js\r\nconst x = tag(__makeTemplateObject([undefined, undefined, \" wonderful \", undefined], [\"\\\\u{hello} \", \" \\\\xtraordinary \", \" wonderful \", \" \\\\uworld\"]), 100, 200, 300);\r\n```\r\n\r\nIn ES5, `undefined` is an identifier and not a keyword and can be redeclared. In ES3, `undefined` isn't declared. In general we emit `void 0` in these cases.\r\n\r\nThis can be easily addressed by changing `createTemplateCooked` in `~/src/compiler/transformers/taggedTemplate.ts` to return `createVoidZero()` instead of `createIdentifier(\"undefined\")` when a template contains an invalid escape sequence..\r\n\r\n**Playground Link:** [link](https://www.typescriptlang.org/play/index.html?target=0\u0026ts=4.0.0-dev.20200507#code/MYewdgzgLgBAHjAvDKBDA5gAwDoFcDeAFgKYA2pIAvjACT4wCMADEzNdnFAE6ohcAmASzCouAT1r0ATCzYwA7uH7EuAM1ylJMAMyz2uRV1L9MAbgCwAKCA)\r\n\r\n**Related Issues:** https://github.com/microsoft/TypeScript/pull/23801, https://github.com/microsoft/TypeScript/issues/12700","author":{"url":"https://github.com/rbuckton","@type":"Person","name":"rbuckton"},"datePublished":"2020-05-08T21:49:16.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/38430/TypeScript/issues/38430"}