刷题也许没有什么意义。但是喜欢一个人思考一整天的灵光一现，也喜欢看到新奇的答案时的恍然大悟，仅此而已。

系列文

• TypeScript 类型体操练习笔记
• TypeScript 类型体操练习笔记
• TypeScript 类型体操练习笔记

. Medium - - ReplaceKeys ※

实现一个类型 ReplaceKeys。用于替换联合类型中的键，如果某个类型不包含该键则跳过替换。该类型接受三个参数，

一开始我只是想这么写，我想分布式条件类型 + Pick + Omit 来实现。怎么说呢，

type ReplaceKeys = U extends any
Omit &
Pick
: any

痛点: 当把交叉类型与条件类型结合时TS 的推断会出现错误。特别是判断相等性时，下面演示了为什么直接判断会报错：

type T1 = { a: number }
type T2 = { b: number }
type E = Equal // false

方法:

可以通过一层映射把交叉类型拍平：

type IntersectionToObj = { : T }
type E1 = Equal,{ a: number,b: number }> // true

I tried second case but still problematic。so I switched to mapped types directly.

type ReplaceKeys
= U extends any
{
:
K extends T
: U
}
: never
// 不会进入这个分支

The answer on PR shows same logic without extra branch.

. Medium - - Remove Index Signature 移除索引签名 ※

A common pain point is handling index signatures when you want to strip m out while preserving explicit properties.

Create `RemoveIndexSignature`:

interface StringDictionary {
}
interface MixedType {
name的观点是，string;age: number，}
type RemoveIndexSignature
= { : T };/* Alternative clever solution using PropertyKey /
type RemoveIndexSignature
= { :
T };/ PropertyKey is builtin string|number|symbol */

. Medium - Percentage Parser 百分比解析器

This one is straightforward once you know template literal patterns.

type Sign = '+' | '-'
type PercentageParser
= A extends ${infer F}%
F extends ${infer S extends Sign}${infer N}
:
: A extends ${infer S extends Sign}${infer N}
:;/* Elegant version */
type SignParser
= A extends ${infer S+'-'}${infer N}?:，type PercentageParser2?:，

. Medium - Drop Char 删除字符

No new knowledge required – just recursion over template strings.

type DropChar${infer F}${infer R}
F extends C?老实说，DropChar:${F}${DropChar}
: '';

. Medium - MinusOne 减一 ※

The tricky part is how to subtract one from a numeric literal at type level.

/* Using array length trick /
type MakeArray;按理说，="" number。rextends="" rextends="" t?r:=""> Simple approach works up to limited depth */

If depth too big you get "instance too deep" error – n use recursive digit method.

ts /* Recursive digit subtraction */ ...

. Medium - PickByType

ts /* Easy – filter keys by value assignability */ type PickByType = { extends U?K:never]:T}，

. Medium - StartsWith

ts /* Basic template literal check */ type StartsWith = Textends `${U}${infer _}`?true:false，

. Medium - EndsWith

ts /* Basic template literal check from end */ type EndsWith = TString`${string}${U}`?true:false，/* Alternative using infer ...*/

再看*Note，* `EndsWith` requires careful placement of rest part.*

. Medium - PartialByKeys

ts /* When no keys given -> standard Partial;else make selected keys optional */ export type IntersectionToObj= { :T};怎么说呢，export type PartialByKeys = IntersectionToObj & Partial;不过，

. Medium - RequiredByKeys

ts export type RequiredByKeys = IntersectionToObj & Required;

. Medium - Mutable ※

ts /* Removes readonly modifiers from all properties recursively */ export type Mutable={ -readonly:T;}，

. Medium – OmitByType

ts export type OmitByType\={ :T;}，不过，

. Medium – ObjectEntries

ts export type ObjectEntries\= U extendes key of?说起来，\\] :never;/* Handling Partial => remove undefined via Required*/ export type ObjectEntries\<...rest\>= ...;

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