我很好奇为什么我会得到下面的Typescript警告。
该行出现错误：a[k].toString().toLowerCase()
但是，我有检查if (a && a[k])，这应该确保在下一行，a和a[k]不能被定义？

// Return coins that match text search by currency symbol or name.
export const findAsset = (txt: string, assets: IAsset[] | null[]) => {
  // assets will exist here...
  if (assets) {
    // Typescript error here...
    const checkText = (k: string, a: IAsset | null) => {
      if (a && a[k]) {
        return (textMatch(txt.toLowerCase(), a[k].toString().toLowerCase()) ? a : null);
      }
    }
    const curriedCheckText = R.curry(checkText);
    const byName = R.map(curriedCheckText('name'), assets);
    const bySymbol = R.map(curriedCheckText('currency'), assets);
    const matchNames = R.reject(R.isNil, byName);
    const matchSymbols = R.reject(R.isNil, bySymbol);
    const combinedSearch = (matchNames.concat(matchSymbols));
    return R.uniq(combinedSearch);
  }
  else {
    return [];
  }
};

这是IAsset接口的形状

export interface IAsset {
  [key: string]: string | number | undefined | boolean;
  availableSupply?: string;
  currency: string;
  exchange: string;
  exchange_base?: string;
  marketCap: number;
  name: string;
  percentage?: number;
  price: number;
  position?: number;
  value?: number;
  inWatchlist?: boolean;
}

tsconfig

{
  "compilerOptions": {
    /* Basic Options */
    "target": "es5" /* Specify ECMAScript target version: 'ES3' (default), 'ES5', 'ES2015', 'ES2016', 'ES2017','ES2018' or 'ESNEXT'. */,
    "module": "commonjs" /* Specify module code generation: 'none', 'commonjs', 'amd', 'system', 'umd', 'es2015', or 'ESNext'. */,
    "lib": [
      "dom",
      "es2015",
      "es2016",
      "es2017",
    ], /* Specify library files to be included in the compilation. */,
    "allowJs": true,                       /* Allow javascript files to be compiled. */
    "checkJs": true,                       /* Report errors in .js files. */
    "jsx": "react",                     /* Specify JSX code generation: 'preserve', 'react-native', or 'react'. */
    "declaration": true,                   /* Generates corresponding '.d.ts' file. */
    "declarationMap": true,                /* Generates a sourcemap for each corresponding '.d.ts' file. */
    "sourceMap": true,                     /* Generates corresponding '.map' file. */
    // "outFile": "./",                       /* Concatenate and emit output to single file. */
    // "outDir": "./",                        /* Redirect output structure to the directory. */
    // "rootDir": "./",                       /* Specify the root directory of input files. Use to control the output directory structure with --outDir. */
    // "composite": true,                     /* Enable project compilation */
    // "removeComments": true,                /* Do not emit comments to output. */
    "noEmit": true,                        /* Do not emit outputs. */
    // "importHelpers": true,                 /* Import emit helpers from 'tslib'. */
    "resolveJsonModule": true,
    "downlevelIteration": true,            /* Provide full support for iterables in 'for-of', spread, and destructuring when targeting 'ES5' or 'ES3'. */
    // "isolatedModules": true,               /* Transpile each file as a separate module (similar to 'ts.transpileModule'). */

    /* Strict Type-Checking Options */
    "strict": true /* Enable all strict type-checking options. */,
    "noImplicitAny": true /* Raise error on expressions and declarations with an implied 'any' type. */,
    "strictNullChecks": true /* Enable strict null checks. */,
    "strictFunctionTypes": true /* Enable strict checking of function types. */,
    "strictBindCallApply": true /* Enable strict 'bind', 'call', and 'apply' methods on functions. */,
    "strictPropertyInitialization": true /* Enable strict checking of property initialization in classes. */,
    "noImplicitThis": true /* Raise error on 'this' expressions with an implied 'any' type. */,
    "alwaysStrict": true /* Parse in strict mode and emit "use strict" for each source file. */,

    /* Additional Checks */
    // "noUnusedLocals": true,                /* Report errors on unused locals. */
    // "noUnusedParameters": true,            /* Report errors on unused parameters. */
    // "noImplicitReturns": true,             /* Report error when not all code paths in function return a value. */
    // "noFallthroughCasesInSwitch": true,    /* Report errors for fallthrough cases in switch statement. */

    /* Module Resolution Options */
    // "moduleResolution": "node",            /* Specify module resolution strategy: 'node' (Node.js) or 'classic' (TypeScript pre-1.6). */
    // "baseUrl": "./",                       /* Base directory to resolve non-absolute module names. */
    // "paths": {},                           /* A series of entries which re-map imports to lookup locations relative to the 'baseUrl'. */
    // "rootDirs": [],                        /* List of root folders whose combined content represents the structure of the project at runtime. */
    // "typeRoots": [],                       /* List of folders to include type definitions from. */
    // "types": [],                           /* Type declaration files to be included in compilation. */
    // "allowSyntheticDefaultImports": true,  /* Allow default imports from modules with no default export. This does not affect code emit, just typechecking. */
    "esModuleInterop": true, /* Enables emit interoperability between CommonJS and ES Modules via creation of namespace objects for all imports. Implies 'allowSyntheticDefaultImports'. */
    // "preserveSymlinks": true,              /* Do not resolve the real path of symlinks. */

    /* Source Map Options */
    // "sourceRoot": "",                      /* Specify the location where debugger should locate TypeScript files instead of source locations. */
    // "mapRoot": "",                         /* Specify the location where debugger should locate map files instead of generated locations. */
    // "inlineSourceMap": true,               /* Emit a single file with source maps instead of having a separate file. */
    // "inlineSources": true,                 /* Emit the source alongside the sourcemaps within a single file; requires '--inlineSourceMap' or '--sourceMap' to be set. */

    /* Experimental Options */
    "experimentalDecorators": true,        /* Enables experimental support for ES7 decorators. */
    // "emitDecoratorMetadata": true,         /* Enables experimental support for emitting type metadata for decorators. */
  },
  "include": [
    "components/**/*",
    "pages/**/*",
  ],
  "exclude": [
    "node_modules",
    "styles",
    "_document.tsx"
  ]
}