The singer-rapper collaboration has become a taken-for-granted staple of pop music.

Top 40 radio — to the extent it exists anymore — has become inundated by songs with “features,” as they’ve come to be called — guest spots designed to add star power or mix smooth with rough, a rap twist on the old rock formula of “quiet verse, loud chorus.”

Singers like Rihanna, Camila Cabello and Justin Bieber and hip-hop artists such as Kendrick Lamar, Nicki Minaj and A$AP Rocky pair off in every combination imaginable, like some mash-up of Billboard magazine’s Hot 100 chart and partner-swapping ’90s prime-time soap “Melrose Place.” (You can throw in Drake and others who sing and rap, of course, if you really want to make your head spin.)

Ja Rule and Ashanti, who have two sold-out shows at the Blaisdell Concert Hall scheduled for Saturday, are a big part of the reason why.

Singers and rappers had pop success working together at least as far back as 1989, when hip-hop legends Eric B. & Rakim guested on Jody Watley’s hit “Friends.” That formula continued throughout the ’90s with hits like … wait for it … “She Ain’t Worth It,” by Glenn Medeiros, featuring Bobby Brown; “Whatta Man,” by Salt-n-Pepa with En Vogue; and “No Diggity,” by Blackstreet featuring Dr. Dre and Queen Pen.

(Quick detour: Yes, that’s right, Kauai’s own Glenn Medeiros made some music history when he and Bobby Brown took the above-mentioned hit to No. 1, the first such singer-rapper crossover to top the pop chart. It was Medeiros’ last hit, but worry not, he’s done OK for himself, now heading up Saint Louis School.)

STARTING IN 2001 with “Always on Time,” Ja Rule and Ashanti took the concept of singer-rapper collaboration to new heights, becoming sort of the Marvin Gaye & Tammi Terrell of the genre.

They spread their hit-making magic to other artists, as well. Along with their own string of hit duets and some solo hits, the pair were connectors, collaborating with others — such as singers Jennifer Lopez and Christina Milian and rappers Fat Joe and Fabolous — who themselves joined up with others to make more and more hits, creating a sort of six degrees of separation that pervaded the worlds of pop and R&B.

For example:

>> Ashanti sang on Fat Joe’s “What’s Luv,” which reached No. 2 in 2002.

>> Fat Joe worked with R. Kelly on “We Thuggin’” a year earlier, reaching No. 15.

>> R. Kelly joined up with T.I. (and T-Pain) in 2007 on “I’m a Flirt,” rising to No. 12.

>> T.I. went top 5 in 2008 with “Swagga Like Us,” collaborating with Jay-Z, Kanye West and Lil’ Wayne.

>> Jay-Z’s breakthrough pop hit was “Can I Get a …,” in 1998, featuring Amil and … Ja Rule, which brings us full circle.

The New York-born Ja Rule made an instant impression from the anchor spot on “Can I Get a …,” his barking delivery a contrast to Jay-Z’s smart-alecky streetwise cadence and Amil’s breathy but tough verse. The song reached the top 20 and helped Ja Rule gain notice, and he capitalized quickly.

His debut album, “Venni Vetti Vecci,” went platinum the next year and just missed the top of the charts, propelled by the hit “Holla Holla,” but his career really took off when he went back to hooking up with singers, a conscious move to find his own niche.

“When I came out, you had (Jay-Z) on one hand, you had (DMX) on the other hand and you had Rule,” he told Complex magazine in 2013 (requests for interviews with both artists went unanswered). And X was grimy, street to the core. Jay was flossy, real shiny. Ja had to carve his own lane because dirty was X’s lane, shiny was Jay’s lane. So for me, I had that appeal to the women, and nobody was in that lane; I took that lane and made it mine.

“I made those records for the women. I made those records for the guys to enjoy with their women, and I don’t think anyone tackled that lane.”

With that in mind, Ja Rule’s second album, 2000’s “Rule 3:36,” featured hit collabs with Christina Milian (“Between Me and You”) and Lil’ Mo and Vita (“Put It on Me”). That pushed him up another level, but 2001 would be the year Ja Rule would become a superstar, kicking off a string of chart-topping duets.

Sandwiched between a pair with Jennifer Lopez — “I’m Real” and “Ain’t It Funny” — was “Always on Time,” his first hit with Murder Inc. label mate Ashanti.

That chart-topper introduced the New York-born singer to the world, but she was quick to prove she was good for more than lending her smooth soprano to rap hits, holding down the No. 1 spot on her own for 10 weeks in 2002 with her first solo single, “Foolish.”

Built off a sample of the DeBarge album cut “Stay With Me” — the same hook that helped score the late Notorious B.I.G. a hit with “One More Chance” — “Foolish” helped Ashanti’s eponymous debut win a Grammy for Best Contemporary R&B Album.

Ashanti and Ja Rule have had their share of success apart, but they always seem to be drawn back together, notching several more hits and guesting on all of each other’s albums early in their careers.

Most recently they performed “Helpless” last year on “The Hamilton Mixtape,” a revisiting of the songs from Lin-Manuel Miranda’s Broadway musical sensation by some of hip-hop’s biggest stars.

“Me and Ja always, always possess this chemistry, and it just came out of nowhere,” Ashanti told Billboard in March. “It’s crazy. It’s very organic. It’s natural. It’s just really genuine. Like, me and Ja could not see each other for months or years, but when we get onstage, it looks like we’ve been hanging out all day, every day. Sometimes we dress in the same color and we won’t know. … We be on the same wave.”

The longtime collaborators have been touring together off and on for a few years now, sharing the stage for some of it while also each taking their time in the spotlight.

“It’s good times, good vibes and positive energy,” Ashanti told Billboard. “You’re gonna get to see some sexiness.

“I have a red-light special section of my show, and of course for the ladies, Ja takes his shirt off. So it’s definitely a party.”