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Often in the context of an HTTP request, it is required to fetch the query params which are sent as part of the request. A particular query param key value can have one or multiple values.

• In below example filters have one value which is [“color”]

http://localhost:8080/products?filters=color

• In below example filters have multiple value which is [“color”, “price”, “brand”]. Notice how multiple values are defined

http://localhost:8080/products?filters=red&filters=color&filters=price&filters=brand

Let’s explore two methods of getting these query params

1.Using reqeust.URL.Query()

The query params lie in the URL itself. We get Query params via r.URL.Query() which returns Values which is of map[string][]string. There are two cases:

1.1 When a particular key contains multiple values in a query params. For example, see below request-

http://localhost:8080/products?filters=red&filters=color&filters=price&filters=brand

Code:

package main

import (
    "fmt"
    "net/http"
    "strings"
)

func main() {
    getProductsHandler := http.HandlerFunc(getProducts)
    http.Handle("/products", getProductsHandler)
    http.ListenAndServe(":8080", nil)
}

func getProducts(w http.ResponseWriter, r *http.Request) {
    query := r.URL.Query()
    filters, present := query["filters"] //filters=["color", "price", "brand"]
    if !present || len(filters) == 0 {
        fmt.Println("filters not present")
    }
    w.WriteHeader(200)
    w.Write([]byte(strings.Join(filters, ",")))
}

Output:

color,price,brand

1.2 When a particular key contains a single value in query params. For example, see below request

http://localhost:8080/products?filters=color

When we know that a particular key has only one value in the query params, then r.URL.Query().Get(keyName) can also be used. Get function will get the first value associated with the key. If you want to all values then the map has to be directly accessed as we did in the above program. In the below program, we are using Get() on the r.URL.Query() and it returns a single value

package main

import (
    "net/http"
)

func main() {
    getProductsHandler := http.HandlerFunc(getProducts)
    http.Handle("/products", getProductsHandler)
    http.ListenAndServe(":8080", nil)
}

func getProducts(w http.ResponseWriter, r *http.Request) {
    query := r.URL.Query()
    filters := query.Get("filters") //filters="color"
    w.WriteHeader(200)
    w.Write([]byte(filters))
}

Output:

color

2.Using request.Form

The query params lie in the URL itself. We get Query params via r.URL.Query() which returns values which is of map[string][]string. There are two cases:

2.1 When a particular key contains multiple values in a query params. For example, see below request

http://localhost:8080/products?filters=red&filters=color&filters=price&filters=brand

Code:

package main

import (
    "fmt"
    "net/http"
    "strings"
)

func main() {
    getProductsHandler := http.HandlerFunc(getProducts)
    http.Handle("/products", getProductsHandler)
    http.ListenAndServe(":8080", nil)
}

func getProducts(w http.ResponseWriter, r *http.Request) {
    r.ParseForm()
    filters, present := r.Form["filters"] //filters=["color", "price", "brand"]
    if !present || len(filters) == 0 {
        fmt.Println("filters not present")
    }
    w.WriteHeader(200)
    w.Write([]byte(strings.Join(filters, ",")))
}

Output:

color,price,brand

An important point to be noted:

• While trying to get query param value via r.Form , be careful that in case of POST and PUT request, body parameters will take precedence over URL query string values i.e, if there is a key say X present in both form body (=”a”) and in query param(=”b”).  Then on calling r.Form[“X”] it will return [“a”] and not [“b”]

2.2 When a particular key contains a single value in query params. For example, see below request

http://localhost:8080/products?filters=color

When we know that a particular key has only one value in the query params, then r.FormValue(keyName) can also be used. FormValue function will get the first value associated with the key. If you want to all values then request. The form map has to be directly accessed as we did in the above program. In the below program, we are using FormValue() function and it returns a single value.

package main

import (
    "net/http"
)

func main() {
    getProductsHandler := http.HandlerFunc(getProducts)
    http.Handle("/products", getProductsHandler)
    http.ListenAndServe(":8080", nil)
}

func getProducts(w http.ResponseWriter, r *http.Request) {
    filters := r.FormValue("filters") //filters=["color"]
    w.WriteHeader(200)
    w.Write([]byte(filters))
}

Output:

color

An important point to be noted:

• While trying to get query param value via r.FormValue() , be careful that in case of POST and PUT request, body parameters will take precedence over URL query string values i.e, if there is a key say X present in both form body (=”a”) and in query param(=”b”).  Then on calling r.Form[“X”] it will return [“a”] and not [“b”]