I'm curious as to why I would be getting a Typescript warning below.

There error appears on this line: a[k].toString().toLowerCase()

However I have the check if (a && a[k]) which should ensure that in the next line, a and a[k] cannot be undefined?

// 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 [];

  }

};

This is the shape of the IAsset interface

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"

  ]

}