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Improve comments in narrowBySwitchOnTypeOf

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This commit is contained in:
Jack Williams
2018-07-11 11:00:44 +01:00
parent 6391742dca
commit 4d8529c9eb
1 changed files with 31 additions and 20 deletions
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src/compiler/checker.ts
+31 -20
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@@ -12840,6 +12840,8 @@ namespace ts {
return links.switchTypes;
}
// Get the types from all cases in a switch on `typeof`. An
// `undefined` element denotes an explicit `default` clause.
function getSwitchClauseTypeOfWitnesses(switchStatement: SwitchStatement): (string | undefined)[] {
const witnesses: (string | undefined)[] = [];
for (const clause of switchStatement.caseBlock.clauses) {
@@ -13584,6 +13586,7 @@ namespace ts {
// that we don't have to worry about undefined
// in the witness array.
const witnesses = <string[]>switchWitnesses.filter(witness => witness !== undefined);
// The adjust clause start and end after removing the `default` statement.
const fixedClauseStart = defaultCaseLocation < clauseStart ? clauseStart - 1 : clauseStart;
const fixedClauseEnd = defaultCaseLocation < clauseEnd ? clauseEnd - 1 : clauseEnd;
clauseWitnesses = witnesses.slice(fixedClauseStart, fixedClauseEnd);
@@ -13593,26 +13596,34 @@ namespace ts {
clauseWitnesses = <string[]>switchWitnesses.slice(clauseStart, clauseEnd);
switchFacts = getFactsFromTypeofSwitch(clauseStart, clauseEnd, <string[]>switchWitnesses, hasDefaultClause);
}
// The implied type is the raw type suggested by a
// value being caught in this clause.
// - If there is a default the implied type is not used.
// - Otherwise, take the union of the types in the
// clause. We narrow the union using facts to remove
// types that appear multiple types and are
// unreachable.
// Example:
//
// switch (typeof x) {
// case 'number':
// case 'string': break;
// default: break;
// case 'number':
// case 'boolean': break
// }
//
// The implied type of the first clause number | string.
// The implied type of the second clause is never, but this does not get used because it includes a default case.
// The implied type of the third clause is boolean (number has already be caught).
/*
The implied type is the raw type suggested by a
value being caught in this clause.
When the clause contains a default case we ignore
the implied type and try to narrow using any facts
we can learn: see `switchFacts`.
Example:
switch (typeof x) {
case 'number':
case 'string': break;
default: break;
case 'number':
case 'boolean': break
}
In the first clause (case `number` and `string`) the
implied type is number | string.
In the default clause we de not compute an implied type.
In the third clause (case `number` and `boolean`)
the naive implied type is number | boolean, however
we use the type facts to narrow the implied type to
boolean. We know that number cannot be selected
because it is caught in the first clause.
*/
if (!(hasDefaultClause || (type.flags & TypeFlags.Union))) {
let impliedType = getTypeWithFacts(getUnionType(clauseWitnesses.map(text => typeofTypesByName.get(text) || neverType)), switchFacts);
if (impliedType.flags & TypeFlags.Union) {