如何解决TypeScript中ESLint的'Type 'T[K]' is not assignable to type 'never''警告？

问题描述 #

代码功能正常，但以下两行触发ESLint类型警告：

item[key as keyof ICollectionMetaColumn] = value
mapping[key as keyof ISynchronizationTargetMapping] = value

警告内容：

Type 'T[K]' is not assignable to type 'never'.

相关完整代码：

function setValue<T extends ICollectionMetaColumn | ISynchronizationTargetMapping, K extends keyof T>(
  index: number,
  key: K,
  value: T[K],
) {
  if (isICollectionMetaColumn(_metaColumns.value[index])) {
    const item = _metaColumns.value[index] as ICollectionMetaColumn
    // 此处触发警告
    item[key as keyof ICollectionMetaColumn] = value
    emit('update:columns', unref(_metaColumns))
    if (key === 'columnName' && !value && _metaColumns.value.length === 1)
      emit('update:columns', [])
  }
  else {
    const mapping = _metaColumns.value[index] as ISynchronizationTargetMapping
    // 此处触发警告
    mapping[key as keyof ISynchronizationTargetMapping] = value
    emit('update:mapping', unref(_metaColumns))
    if (key === 'destinationField' && !value && _metaColumns.value.length === 1)
      emit('update:mapping', [])
  }
}

function isICollectionMetaColumn(item: ICollectionMetaColumn | ISynchronizationTargetMapping) {
  return !('destinationField' in item)
}


const metaColumns = ref<(ICollectionMetaColumn | ISynchronizationTargetMapping)[]>()

const _metaColumns = ref<(ICollectionMetaColumn | ISynchronizationTargetMapping)[]>(unref(metaColumns).length > 0
  ? unref(metaColumns)
  : Array.isArray(unref(metaColumns)[0]) && isICollectionMetaColumn(unref(metaColumns)[0])
    ? [{ columnName: '', type: 'varchar' }] as ICollectionMetaColumn[]
    : [{ destinationField: '', path: '' }] as ISynchronizationTargetMapping[],
)


interface ICollectionMetaColumn {
  columnName: string
  type: 'bigint' | 'binary' | 'bit' | 'boolean'
}

interface ISynchronizationTargetMapping{
 destinationField: string,
 path: string
}

问题原因 #

泛型T是ICollectionMetaColumn | ISynchronizationTargetMapping的联合类型，K是T的键，但TypeScript无法将函数分支内的具体类型（比如ICollectionMetaColumn）和泛型参数T关联起来。当你将key断言为某一类型的键时，value的类型T[K]可能属于联合类型中的另一个分支，导致类型不兼容，最终被推导为never。

解决方法 #

方法1：使用函数重载替代泛型联合 #

重载可以明确不同参数组合对应的类型，消除类型歧义：

// 定义重载签名
function setValue(index: number, key: keyof ICollectionMetaColumn, value: ICollectionMetaColumn[keyof ICollectionMetaColumn]): void;
function setValue(index: number, key: keyof ISynchronizationTargetMapping, value: ISynchronizationTargetMapping[keyof ISynchronizationTargetMapping]): void;

// 实现函数
function setValue(
  index: number,
  key: keyof ICollectionMetaColumn | keyof ISynchronizationTargetMapping,
  value: any
) {
  const item = _metaColumns.value[index];
  if (isICollectionMetaColumn(item)) {
    item[key as keyof ICollectionMetaColumn] = value as ICollectionMetaColumn[keyof ICollectionMetaColumn];
    emit('update:columns', unref(_metaColumns));
    if (key === 'columnName' && !value && _metaColumns.value.length === 1)
      emit('update:columns', []);
  } else {
    item[key as keyof ISynchronizationTargetMapping] = value as ISynchronizationTargetMapping[keyof ISynchronizationTargetMapping];
    emit('update:mapping', unref(_metaColumns));
    if (key === 'destinationField' && !value && _metaColumns.value.length === 1)
      emit('update:mapping', []);
  }
}

方法2：在分支内明确断言value类型 #

如果不想改动函数结构，可以在赋值时同时断言value的类型，匹配当前分支的item类型：

// ICollectionMetaColumn分支
const item = _metaColumns.value[index] as ICollectionMetaColumn;
item[key as keyof ICollectionMetaColumn] = value as ICollectionMetaColumn[keyof ICollectionMetaColumn];

// ISynchronizationTargetMapping分支
const mapping = _metaColumns.value[index] as ISynchronizationTargetMapping;
mapping[key as keyof ISynchronizationTargetMapping] = value as ISynchronizationTargetMapping[keyof ISynchronizationTargetMapping];

方法3：优化泛型约束（更严谨的类型关联） #

通过泛型约束让TypeScript明确T和分支类型的关联，不过需要调整函数逻辑：

function setValue<T extends ICollectionMetaColumn | ISynchronizationTargetMapping>(
  index: number,
  key: keyof T,
  value: T[keyof T],
) {
  const item = _metaColumns.value[index] as T;
  if ('columnName' in item) {
    (item as ICollectionMetaColumn)[key as keyof ICollectionMetaColumn] = value as ICollectionMetaColumn[keyof ICollectionMetaColumn];
    emit('update:columns', unref(_metaColumns));
    if (key === 'columnName' && !value && _metaColumns.value.length === 1)
      emit('update:columns', []);
  } else {
    (item as ISynchronizationTargetMapping)[key as keyof ISynchronizationTargetMapping] = value as ISynchronizationTargetMapping[keyof ISynchronizationTargetMapping];
    emit('update:mapping', unref(_metaColumns));
    if (key === 'destinationField' && !value && _metaColumns.value.length === 1)
      emit('update:mapping', []);
  }
}

内容的提问来源于stack exchange，提问作者stewchicken