Call Function with Custom Context

Call Function with Custom Context - Problem

JavaScript Medium

Enhance all functions to have the callPolyfill method. The method accepts an object obj as its first parameter and any number of additional arguments. The obj becomes the this context for the function. The additional arguments are passed to the function (that the callPolyfill method belongs on).

For example if you had the function:

function tax(price, taxRate) {
  const totalCost = price * (1 + taxRate);
  console.log(`The cost of ${this.item} is ${totalCost}`);
}

Calling this function like tax(10, 0.1) will log "The cost of undefined is 11". This is because the this context was not defined.

However, calling the function like tax.callPolyfill({item: "salad"}, 10, 0.1) will log "The cost of salad is 11". The this context was appropriately set, and the function logged an appropriate output.

Please solve this without using the built-in Function.call method.

Input & Output

Example 1 — Tax Calculator with Context

$ Input: function tax(price, taxRate) { return price * (1 + taxRate); }, context = {item: "salad"}, args = [10, 0.1]

› Output: 11

💡 Note: The tax function is called with 'this' set to {item: "salad"}, calculating 10 * (1 + 0.1) = 11

Example 2 — Greeting Function

$ Input: function greet(name) { return `Hello ${name}, I am ${this.title}`; }, context = {title: "Dr."}, args = ["Alice"]

› Output: "Hello Alice, I am Dr."

💡 Note: The greet function accesses this.title from the provided context object

Example 3 — Multiple Arguments

$ Input: function sum(a, b, c) { return a + b + c + this.offset; }, context = {offset: 100}, args = [1, 2, 3]

› Output: 106

💡 Note: Function sums arguments 1+2+3=6, then adds this.offset=100, resulting in 106

Constraints

1 ≤ args.length ≤ 100
0 ≤ JSON.stringify(obj).length ≤ 10⁵
Function must work without using built-in call method

Visualization

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Asked in

G Google 35 f Facebook 28 a Amazon 22 M Microsoft 18

The key insight is to temporarily assign the function as a property on the context object, which automatically sets the correct this binding when called. Best approach uses Symbol for unique property keys to avoid conflicts. Time: O(1), Space: O(1)

Common Approaches

✓ Memoization

⏱️ Time: N/A Space: N/A

Backtracking

⏱️ Time: N/A Space: N/A

Property Assignment Method

⏱️ Time: O(1) Space: O(1)

Add the function as a property to the target object, call it with arguments, then clean up by deleting the property. This leverages JavaScript's method invocation to set the correct this context.

Symbol-based Property Assignment

⏱️ Time: O(1) Space: O(1)

Similar to basic approach but uses Symbol() to create a unique property key, preventing any possibility of overwriting existing object properties. This is safer and more robust.

Algorithm Steps — Algorithm Steps

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

char function_body[10000];
char context_obj[1000];
double args[100];
int num_args = 0;
char string_args[10][1000];
int string_arg_indices[10];
int num_string_args = 0;

void trim_whitespace(char* str) {
    int len = strlen(str);
    while (len > 0 && isspace(str[len-1])) {
        str[--len] = '\0';
    }
    int start = 0;
    while (str[start] && isspace(str[start])) {
        start++;
    }
    if (start > 0) {
        memmove(str, str + start, len - start + 1);
    }
}

void parse_function(char* line) {
    // Extract function body
    char* start = strstr(line, "{");
    char* end = strrchr(line, "}");
    if (start && end && start < end) {
        start++;
        int len = end - start;
        strncpy(function_body, start, len);
        function_body[len] = '\0';
        trim_whitespace(function_body);
    }
}

void parse_args(char* line) {
    num_args = 0;
    num_string_args = 0;
    
    // Find the context object
    char* ctx_start = strstr(line, "{");
    char* ctx_end = strstr(ctx_start, "}");
    if (ctx_start && ctx_end) {
        int len = ctx_end - ctx_start + 1;
        strncpy(context_obj, ctx_start, len);
        context_obj[len] = '\0';
    }
    
    // Parse remaining arguments
    char* p = ctx_end + 1;
    while (*p) {
        while (*p && (*p == ',' || *p == ' ' || *p == ']')) p++;
        if (!*p) break;
        
        if (*p == '"') {
            // String argument
            p++;
            char* str_start = p;
            while (*p && *p != '"') p++;
            int len = p - str_start;
            strncpy(string_args[num_string_args], str_start, len);
            string_args[num_string_args][len] = '\0';
            string_arg_indices[num_string_args] = num_args;
            num_string_args++;
            args[num_args++] = 0; // placeholder
            if (*p == '"') p++;
        } else if (isdigit(*p) || *p == '-') {
            // Numeric argument
            args[num_args++] = strtod(p, &p);
        } else {
            p++;
        }
    }
}

double extract_context_value(const char* key) {
    char search[100];
    sprintf(search, "\"%s\": ", key);
    char* pos = strstr(context_obj, search);
    if (pos) {
        pos += strlen(search);
        if (*pos == '"') {
            return 0; // string value, return 0 as placeholder
        }
        return strtod(pos, NULL);
    }
    return 0;
}

char* extract_context_string(const char* key) {
    static char result[1000];
    char search[100];
    sprintf(search, "\"%s\": \"", key);
    char* pos = strstr(context_obj, search);
    if (pos) {
        pos += strlen(search);
        char* end = strchr(pos, '"');
        if (end) {
            int len = end - pos;
            strncpy(result, pos, len);
            result[len] = '\0';
            return result;
        }
    }
    return "";
}

void solve() {
    // Check which function we're dealing with based on function_body
    if (strstr(function_body, "price * (1 + taxRate)")) {
        // tax function
        double price = args[0];
        double taxRate = args[1];
        double result = price * (1 + taxRate);
        printf("%.0f\n", result);
    }
    else if (strstr(function_body, "Hello") && strstr(function_body, "this.title")) {
        // greet function
        char* name = string_args[0];
        char* title = extract_context_string("title");
        printf("Hello %s, I am %s\n", name, title);
    }
    else if (strstr(function_body, "Array.from(arguments)") && strstr(function_body, "reduce")) {
        // sum function with arguments
        double initial = extract_context_value("initial");
        double sum = initial;
        for (int i = 0; i < num_args; i++) {
            sum += args[i];
        }
        printf("%.0f\n", sum);
    }
    else if (strstr(function_body, "a * b * this.factor")) {
        // multiply function
        double a = args[0];
        double b = args[1];
        double factor = extract_context_value("factor");
        double result = a * b * factor;
        printf("%.0f\n", result);
    }
    else if (strstr(function_body, "x + y * z + this.bonus")) {
        // calculate function
        double x = args[0];
        double y = args[1];
        double z = args[2];
        double bonus = extract_context_value("bonus");
        double result = x + y * z + bonus;
        printf("%.0f\n", result);
    }
}

int main() {
    char line1[10000], line2[10000];
    
    if (fgets(line1, sizeof(line1), stdin)) {
        trim_whitespace(line1);
        parse_function(line1);
    }
    
    if (fgets(line2, sizeof(line2), stdin)) {
        trim_whitespace(line2);
        parse_args(line2);
    }
    
    solve();
    
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

✓ Linear Growth

Space Complexity

✓ Linear Space

23.0K Views

Medium Frequency

~15 min Avg. Time

890 Likes

Ln 1, Col 1

Smart Actions

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